[clang] [Clang][HIP] Deprecate the AMDGCN_WAVEFRONT_SIZE macros (PR #112849)

[Shilei Tian via cfe-commits]

shiltian wrote:

> > > Just a heads up, this change is gonna break some OpenMP tests, specifically those prefix with `ompx`.
> > 
> > 
> > Just to clarify, adding the deprecation warning will break them, or the eventual, as-of-yet not-scheduled, removal, will?
> 
> On my system, the deprecation warning did not break any lit tests (nor tests from our internal testing). The `ompx` tests use the macro and will break once we actually remove it.

Oh didn’t notice this PR was actually to just warn it instead of entirely removing it. Sorry for the notice.

https://github.com/llvm/llvm-project/pull/112849