[Serialization] Ignore missing potentially optional dependencies in getImportedModules

The service `ModuleFile::getImportedModules` is called after the
dependency loading logic runs. We can trust this previous logic to have
correctly loaded optional dependencies. In `getImportedModules` we can
just ignore such missing dependencies.

rdar://115372249

Author: xymus

lib/Serialization/ModuleFile.cpp

@@ -496,20 +496,20 @@ void ModuleFile::getImportedModules(SmallVectorImpl<ImportedModule> &results,
         continue;
 
     } else if (dep.isImplementationOnly()) {
-      if (!filter.contains(ModuleDecl::ImportFilterKind::ImplementationOnly))
+      // Pretend we didn't have potentially optional imports if we weren't
+      // originally asked to load it.
+      if (!filter.contains(ModuleDecl::ImportFilterKind::ImplementationOnly) ||
+          !dep.isLoaded())
         continue;
-      if (!dep.isLoaded()) {
-        // Pretend we didn't have this import if we weren't originally asked to
-        // load it.
-        continue;
-      }
 
     } else if (dep.isInternalOrBelow()) {
-      if (!filter.contains(ModuleDecl::ImportFilterKind::InternalOrBelow))
+      if (!filter.contains(ModuleDecl::ImportFilterKind::InternalOrBelow) ||
+          !dep.isLoaded())
         continue;
 
     } else if (dep.isPackageOnly()) {
-      if (!filter.contains(ModuleDecl::ImportFilterKind::PackageOnly))
+      if (!filter.contains(ModuleDecl::ImportFilterKind::PackageOnly) ||
+          !dep.isLoaded())
         continue;
 
     } else {

test/Serialization/access-level-import-dependencies.swift

@@ -123,6 +123,7 @@ import PrivateDep
 // RUN:   -enable-library-evolution -enable-testing \
 // RUN:   -enable-experimental-feature AccessLevelOnImport
 // RUN: %target-swift-frontend -typecheck %t/ExporterClient.swift -I %t \
+// RUN:   -index-system-modules -index-ignore-stdlib -index-store-path %t/idx \
 // RUN:   -Rmodule-loading 2>&1 | %FileCheck -check-prefixes=CHECK-EXPORTER,HIDDEN-DEP %s
 // CHECK-EXPORTER: 'InternalDep' has an ignored transitive dependency on 'HiddenDep'