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

[via cfe-commits]

zmodem wrote:

> Having a `-Wdeprecated-include-gch` looks good to me.

But do we intend to remove it later? That will break the drop-in ability of Clang, and probably a lot of people's builds.

> The inconvenience probing .gch happens when GCC and Clang .pch files are mismatched

To solve this, couldn't the driver peek at the magic at the start of the file or something like that?

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