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

[Nico Weber via cfe-commits]

nico wrote:

Can we add a dedicated `-Wdeprecated-gch` diag group or something for this? This is a very disruptive change, and it'd be good if we didn't have to disable all deprecation warnings while we rearrange things.

Reading the linked bugs, it's not clear that removing this would've helped all these issues all that much and it introduces a bunch of toil. So maybe we don't want to do this at all? But if we do, please put it behind a dedicated warning flag.

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