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

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zmodem wrote:

As an archaeological note, the gch probing was added a long time ago in a2aedc69435dd430925e77022cbd26cc51e30982 with the motivation

> It's wonky, but we include looking for .gch so we can support seamless replacement into a build system already set up to be generating .gch files.

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