Initialize clang::CodeGenOptions.DebugExtRefs in ClangImporter.

This fixes a race condition in parallel builds. DebugExtRefs is part of the
Clang module hash. ObjectFilePCHContainerOperations has its own CodenOpts object
which always sets DebugExtRefs=true, so the module has could differ depending on
whether a module was built as a top-level module or as a dependency of another
module.

This fixes the SwiftREPL tests.

rdar://86279876

Author: adrian-prantl

lib/ClangImporter/ClangImporter.cpp

@@ -1130,6 +1130,10 @@ ClangImporter::create(ASTContext &ctx,
 
   // Create a compiler instance.
   {
+    // The Clang modules produced by ClangImporter are always embedded in an
+    // ObjectFilePCHContainer and contain -gmodules debug info.
+    importer->Impl.Invocation->getCodeGenOpts().DebugTypeExtRefs = true;
+
     auto PCHContainerOperations =
       std::make_shared<clang::PCHContainerOperations>();
     PCHContainerOperations->registerWriter(