[DO NOT SUBMIT] Add an -isystem path only to non-builtin include paths.

[ruiu]

Summary

This is a test.

[ruiu]

• address review comments (demo)

clang/lib/Frontend/CompilerInvocation.cpp

for (const Arg *A : Args.filtered(OPT_objcxx_isystem))
     Opts.AddPath(A->getValue(), frontend::ObjCXXSystem, false, true);
 
   // Add the internal paths from a driver that detects standard include paths.
   for (const Arg *A :
        Args.filtered(OPT_internal_isystem, OPT_internal_externc_isystem)) {
     frontend::IncludeDirGroup Group = frontend::System;
     if (A->getOption().matches(OPT_internal_externc_isystem))
       Group = frontend::ExternCSystem;
+
+    SmallString<128> P(D.ResourceDir);
+    llvm::sys::path::append(P, "include");
+
+    if (StringRef(A->getValue()) == P)
       Opts.AddPath(A->getValue(), Group, false, true);
+    else
+      Opts.AddPath(A->getValue(), Group, false, false);
   }
 
   // Add the path prefixes which are implicitly treated as being system headers.
   for (const Arg *A :
        Args.filtered(OPT_system_header_prefix, OPT_no_system_header_prefix))
     Opts.AddSystemHeaderPrefix(
         A->getValue(), A->getOption().matches(OPT_system_header_prefix));
 
for (const std::string &File : CI.getHeaderSearchOpts().VFSOverlayFiles) {
     }
 
     IntrusiveRefCntPtr<vfs::FileSystem> FS = vfs::getVFSFromYAML(
         std::move(Buffer.get()), /*DiagHandler*/ nullptr, File);
     if (!FS.get()) {
       Diags.Report(diag::err_invalid_vfs_overlay) << File;
       return IntrusiveRefCntPtr<vfs::FileSystem>();
     }
-    Overlay->pushOverlay(FS);
-  }
   return Overlay;
 }
 } // end namespace clang