[Sema] @_exported imports export @_spi decls too

Enable transitive imports of all SPI groups through @_exported imports.
This brings to SPI the same behavior that we have for API.

```
// Module A
@_spi(S) public func foo() {}

// Module B
@_exported import A

// Module C
@_spi(S) import B

foo() // SPI imported through the reexport of A from B
```

rdar://101566534

Author: xymus

lib/AST/Module.cpp

@@ -2849,9 +2849,11 @@ canBeUsedForCrossModuleOptimization(DeclContext *ctxt) const {
 void SourceFile::lookupImportedSPIGroups(
                         const ModuleDecl *importedModule,
                         llvm::SmallSetVector<Identifier, 4> &spiGroups) const {
+  auto &imports = getASTContext().getImportCache();
   for (auto &import : *Imports) {
     if (import.options.contains(ImportFlags::SPIAccessControl) &&
-        importedModule == import.module.importedModule) {
+        (importedModule == import.module.importedModule ||
+         imports.isImportedBy(importedModule, import.module.importedModule))) {
       spiGroups.insert(import.spiGroups.begin(), import.spiGroups.end());
     }
   }

lib/Serialization/SerializedModuleLoader.cpp

@@ -1488,11 +1488,13 @@ void SerializedASTFile::lookupImportedSPIGroups(
                         const ModuleDecl *importedModule,
                         llvm::SmallSetVector<Identifier, 4> &spiGroups) const {
   auto M = getParentModule();
+  auto &imports = M->getASTContext().getImportCache();
   for (auto &dep : File.Dependencies) {
     if (!dep.Import.hasValue())
       continue;
 
-    if (dep.Import->importedModule == importedModule) {
+    if (dep.Import->importedModule == importedModule ||
+        imports.isImportedBy(importedModule, dep.Import->importedModule)) {
       spiGroups.insert(dep.spiGroups.begin(), dep.spiGroups.end());
     }
   }

test/SPI/reexported-spi-groups.swift

@@ -0,0 +1,148 @@
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+
+/// Build lib defining SPIs
+// RUN: %target-swift-frontend -emit-module %t/Exported.swift \
+// RUN:   -module-name Exported -swift-version 5 \
+// RUN:   -enable-library-evolution \
+// RUN:   -emit-module-path %t/Exported.swiftmodule \
+// RUN:   -emit-module-interface-path %t/Exported.swiftinterface \
+// RUN:   -emit-private-module-interface-path %t/Exported.private.swiftinterface
+// RUN: %target-swift-typecheck-module-from-interface(%t/Exported.swiftinterface)
+// RUN: %target-swift-typecheck-module-from-interface(%t/Exported.private.swiftinterface) -module-name Exported
+
+/// Build lib reexporting SPIs
+// RUN: %target-swift-frontend -emit-module %t/Exporter.swift \
+// RUN:   -module-name Exporter -swift-version 5 -I %t \
+// RUN:   -enable-library-evolution \
+// RUN:   -emit-module-path %t/Exporter.swiftmodule \
+// RUN:   -emit-module-interface-path %t/Exporter.swiftinterface \
+// RUN:   -emit-private-module-interface-path %t/Exporter.private.swiftinterface
+// RUN: %target-swift-typecheck-module-from-interface(%t/Exporter.swiftinterface) -I %t
+// RUN: %target-swift-typecheck-module-from-interface(%t/Exporter.private.swiftinterface) -module-name Exporter -I %t
+
+/// Build lib not reexporting SPIs (a normal import)
+// RUN: %target-swift-frontend -emit-module %t/NonExporter.swift \
+// RUN:   -module-name NonExporter -swift-version 5 -I %t \
+// RUN:   -enable-library-evolution \
+// RUN:   -emit-module-path %t/NonExporter.swiftmodule \
+// RUN:   -emit-module-interface-path %t/NonExporter.swiftinterface \
+// RUN:   -emit-private-module-interface-path %t/NonExporter.private.swiftinterface
+// RUN: %target-swift-typecheck-module-from-interface(%t/NonExporter.swiftinterface) -I %t
+// RUN: %target-swift-typecheck-module-from-interface(%t/NonExporter.private.swiftinterface) -module-name NonExporter -I %t
+
+/// Build client of transitive SPIs and its swiftinterfaces
+// RUN: %target-swift-frontend -emit-module %t/ClientLib.swift \
+// RUN:   -module-name ClientLib -swift-version 5 -I %t \
+// RUN:   -enable-library-evolution \
+// RUN:   -emit-module-path %t/ClientLib.swiftmodule \
+// RUN:   -emit-module-interface-path %t/ClientLib.swiftinterface \
+// RUN:   -emit-private-module-interface-path %t/ClientLib.private.swiftinterface
+// RUN: %target-swift-typecheck-module-from-interface(%t/ClientLib.swiftinterface) -I %t
+// RUN: %target-swift-typecheck-module-from-interface(%t/ClientLib.private.swiftinterface) -module-name ClientLib -I %t
+
+/// Test diagnostics of a multifile client
+// RUN: %target-swift-frontend -typecheck \
+// RUN:   %t/Client_FileA.swift %t/Client_FileB.swift\
+// RUN:   -swift-version 5 -I %t -verify
+
+/// Test that SPIs don't leak when not reexported
+// RUN: %target-swift-frontend -typecheck \
+// RUN:   %t/NonExporterClient.swift \
+// RUN:   -swift-version 5 -I %t -verify
+
+/// Test diagnostics against private swiftinterfaces
+// RUN: rm %t/Exported.swiftmodule %t/Exporter.swiftmodule
+// RUN: %target-swift-frontend -typecheck \
+// RUN:   %t/Client_FileA.swift %t/Client_FileB.swift\
+// RUN:   -swift-version 5 -I %t -verify
+
+/// Test diagnostics against public swiftinterfaces
+// RUN: rm %t/Exported.private.swiftinterface %t/Exporter.private.swiftinterface
+// RUN: %target-swift-frontend -typecheck \
+// RUN:   %t/PublicClient.swift \
+// RUN:   -swift-version 5 -I %t -verify
+
+
+//--- Exported.swift
+
+public func exportedPublicFunc() {}
+
+@_spi(X) public func exportedSpiFunc() {}
+
+@_spi(X) public struct ExportedSpiType {}
+
+//--- Exporter.swift
+
+@_exported import Exported
+
+@_spi(X) public func exporterSpiFunc() {}
+
+//--- NonExporter.swift
+
+@_spi(X) import Exported
+
+@_spi(X) public func exporterSpiFunc() {}
+
+//--- ClientLib.swift
+
+@_spi(X) import Exporter
+
+public func clientA() {
+    exportedPublicFunc()
+    exportedSpiFunc()
+    exporterSpiFunc()
+}
+
+@_spi(X) public func spiUseExportedSpiType(_ a: ExportedSpiType) {}
+
+//--- Client_FileA.swift
+
+@_spi(X) import Exporter
+
+public func clientA() {
+    exportedPublicFunc()
+    exportedSpiFunc()
+    exporterSpiFunc()
+}
+
+@inlinable
+public func inlinableClient() {
+    exportedPublicFunc()
+    exportedSpiFunc() // expected-error {{global function 'exportedSpiFunc()' cannot be used in an '@inlinable' function because it is an SPI imported from 'Exported'}}
+    exporterSpiFunc() // expected-error {{global function 'exporterSpiFunc()' cannot be used in an '@inlinable' function because it is an SPI imported from 'Exporter'}}
+}
+
+@_spi(X) public func spiUseExportedSpiType(_ a: ExportedSpiType) {}
+
+public func publicUseExportedSpiType(_ a: ExportedSpiType) {} // expected-error {{cannot use struct 'ExportedSpiType' here; it is an SPI imported from 'Exported'}}
+
+//--- Client_FileB.swift
+
+import Exporter
+
+public func clientB() {
+    exportedPublicFunc()
+    exportedSpiFunc() // expected-error {{cannot find 'exportedSpiFunc' in scope}}
+    exporterSpiFunc() // expected-error {{cannot find 'exporterSpiFunc' in scope}}
+}
+
+//--- NonExporterClient.swift
+
+@_spi(X) import NonExporter
+
+public func client() {
+    exportedPublicFunc() // expected-error {{cannot find 'exportedPublicFunc' in scope}}
+    exportedSpiFunc() // expected-error {{cannot find 'exportedSpiFunc' in scope}}
+    exporterSpiFunc()
+}
+
+//--- PublicClient.swift
+
+@_spi(X) import Exporter // expected-warning {{'@_spi' import of 'Exporter' will not include any SPI symbols}}
+
+public func client() {
+    exportedPublicFunc()
+    exportedSpiFunc() // expected-error {{cannot find 'exportedSpiFunc' in scope}}
+    exporterSpiFunc() // expected-error {{cannot find 'exporterSpiFunc' in scope}}
+}