[Serialization] Support loading an aliased module with an underlying module

Importing a module using an alias (set with -module-alias) fails if the
imported module has an underlying clang module.

rdar://102217790

Author: xymus

lib/Frontend/ModuleInterfaceLoader.cpp

@@ -614,7 +614,8 @@ class ModuleInterfaceLoaderImpl {
     StringRef inParentDirName =
       path::filename(path::parent_path(interfacePath));
     if (path::extension(inParentDirName) == ".swiftmodule") {
-      assert(path::stem(inParentDirName) == moduleName);
+      assert(path::stem(inParentDirName) ==
+             ctx.getRealModuleName(ctx.getIdentifier(moduleName)).str());
       path::append(scratch, inParentDirName);
     }
     path::append(scratch, path::filename(modulePath));

lib/Serialization/Deserialization.cpp

@@ -2359,8 +2359,9 @@ ModuleDecl *ModuleFile::getModule(ImportPath::Module name,
     return getContext().TheBuiltinModule;
 
   // FIXME: duplicated from ImportResolver::getModule
+  Identifier parentName = FileContext->getParentModule()->getName();
   if (name.size() == 1 &&
-      name.front().Item == FileContext->getParentModule()->getName()) {
+      name.front().Item == getContext().getRealModuleName(parentName)) {
     if (!UnderlyingModule && allowLoading) {
       auto importer = getContext().getClangModuleLoader();
       assert(importer && "no way to import shadowed module");

test/ModuleInterface/ambiguous-aliases-workaround.swift

@@ -18,6 +18,14 @@
 // RUN:     -alias-module-names-in-module-interface
 // RUN: %target-swift-typecheck-module-from-interface(%t/AmbiguousClientName.swiftinterface) -I%t
 
+// RUN: %target-swift-frontend -emit-module -module-name OverlayClient \
+// RUN:     -swift-version 5 -enable-library-evolution \
+// RUN:     -o %t/OverlayClient.swiftmodule \
+// RUN:     -emit-module-interface-path %t/OverlayClient.swiftinterface \
+// RUN:     %t/OverlayClient.swift -I%t \
+// RUN:     -alias-module-names-in-module-interface
+// RUN: %target-swift-typecheck-module-from-interface(%t/OverlayClient.swiftinterface) -I%t
+
 //--- module.modulemap
 module AmbiguousClientName {
     header "AmbiguousClientName.h"
@@ -64,3 +72,10 @@ public func refToNestedInLib(_ a: AmbiguousLib.Nested) {}
 public func refToStdlib(_ a: Swift.Int) {}
 public func refToUnderlying(_ a: UnderlyingType) {}
 public func refToC(_ a: CType) {}
+
+//--- OverlayClient.swift
+
+import AmbiguousClientName
+
+public func refToImportedType(_ a: SomeType) {}
+public func refToImportedUnderlying(_ a: UnderlyingType) {}