Inherit parent's extra Clang arguments when creating an interface build sub-invocation.

We would like to make sure they get propagated to downstream transitive Clang module dependencies also.

Resolves rdar://101923680

lib/Frontend/ModuleInterfaceLoader.cpp

@@ -1597,18 +1597,11 @@ InterfaceSubContextDelegateImpl::InterfaceSubContextDelegateImpl(
   subClangImporterOpts.DetailedPreprocessingRecord =
     clangImporterOpts.DetailedPreprocessingRecord;
 
-  // We need to add these extra clang flags because explicit module building
-  // related flags are all there: -fno-implicit-modules, -fmodule-map-file=,
-  // and -fmodule-file=.
-  // If we don't add these flags, the interface will be built with implicit
-  // PCMs.
-  // FIXME: With Implicit Module Builds, if sub-invocations inherit `-fmodule-map-file=` options,
-  // those modulemaps become File dependencies of all downstream PCMs and their depending Swift
-  // modules, triggering unnecessary re-builds. We work around this by only inheriting these options
-  // when building with explicit modules. While this problem will not manifest with Explicit Modules
-  // (which do not use the ClangImporter to build PCMs), we may still need a better way to
-  // decide which options must be inherited here.
-  if (LoaderOpts.disableImplicitSwiftModule) {
+  // If the compiler has been asked to be strict with ensuring downstream dependencies
+  // get the parent invocation's context, or this is an Explicit build, inherit the
+  // extra Clang arguments also.
+  if (LoaderOpts.strictImplicitModuleContext || LoaderOpts.disableImplicitSwiftModule) {
+    // Inherit any clang-specific state of the compilation (macros, clang flags, etc.)
     subClangImporterOpts.ExtraArgs = clangImporterOpts.ExtraArgs;
     for (auto arg : subClangImporterOpts.ExtraArgs) {
       GenericArgs.push_back("-Xcc");