[clang] [Driver] -include: deprecate probing .gch (PR #67084)

[Fangrui Song via cfe-commits]

MaskRay 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.

Sounds good. Switched to a `-Wdeprecated` diagnostic.

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