[clang] [C23] Accept an _Atomic underlying type (PR #147802)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> > avior due to the silent stripping. Given that an atomic type is not the same as its underlying type (in terms of ABI or semantics) I think we should diagnose the behavior with at least a warning. I could even be convinced it should be a warning which defaults to an error because this is just weird.
> 
> Yeah, I 100% agree with that. Warn-as-error is reasonable here IMO (as are ANY qualifiers?, but more so for atomic)

Would you like me to diagnose any qualifier? And turn it into warning-as-error?

https://github.com/llvm/llvm-project/pull/147802