[Serialization] Gate Clang type (de)serialization behind UseClangFunctionTypes.

Author: varungandhi-apple

lib/Serialization/Deserialization.cpp

@@ -5799,6 +5799,9 @@ class SwiftToClangBasicReader :
 
 llvm::Expected<const clang::Type *>
 ModuleFile::getClangType(ClangTypeID TID) {
+  if (!getContext().LangOpts.UseClangFunctionTypes)
+    return nullptr;
+
   if (TID == 0)
     return nullptr;
 

lib/Serialization/Serialization.cpp

@@ -4140,7 +4140,10 @@ class Serializer::TypeSerializer : public TypeVisitor<TypeSerializer> {
     using namespace decls_block;
 
     auto resultType = S.addTypeRef(fnTy->getResult());
-    auto clangType = S.addClangTypeRef(fnTy->getClangTypeInfo().getType());
+    auto clangType =
+      S.getASTContext().LangOpts.UseClangFunctionTypes
+      ? S.addClangTypeRef(fnTy->getClangTypeInfo().getType())
+      : ClangTypeID(0);
 
     unsigned abbrCode = S.DeclTypeAbbrCodes[FunctionTypeLayout::Code];
     FunctionTypeLayout::emitRecord(S.Out, S.ScratchRecord, abbrCode,