[clang] [C99] Claim conformance to WG14 N717 (PR #87228)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> Thanks Aaron for putting up with all my offline requests. LGTM

Thank you for all the excellent suggestions, this was a lot harder to test correctly than I realized at first.

https://github.com/llvm/llvm-project/pull/87228