[clang] [Driver] -include: do not probe .gch (PR #67084)

[Jan Svoboda via cfe-commits]

https://github.com/jansvoboda11 requested changes to this pull request.

Xcode is actively using this feature: the .pch extension is used for headers that are to be precompiled, the .pch.gch extension is used for the actual precompiled header, and Clang is expected to expand the given -include X.pch argument into -include-pch X.pch.gch.

We're okay with changing the build system to explicitly pass -include-pch X.pch.gch to the compiler, but I think it would be more prudent for Clang to emit a deprecation warning for now and only remove this feature (land this patch) after the next upstream release is cut.

https://github.com/llvm/llvm-project/pull/67084