[clang] [C99] Claim conformance to WG14 N590 (PR #86985)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

I put this up for review because I wanted the extra testing on Precommit CI for Linux (I had locally verified the test on Windows myself). Going to merge without review because this is just adding test coverage.

https://github.com/llvm/llvm-project/pull/86985