[clang] [C2y] Claim conformance to WG14 N3254 (PR #97581)

[John McCall via cfe-commits]

rjmccall wrote:

Yes, I agree that we can reasonably claim to support this for the indefinite future.  This is something we would've seen ourselves as practically constrained from exploiting anyway, effectively making it well-defined even if the standard didn't act.

https://github.com/llvm/llvm-project/pull/97581