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

[Jan Svoboda via cfe-commits]

jansvoboda11 wrote:

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