[clang] [C99] Claim conformance for _Complex support (PR #88161)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

I'm not convinced I have sufficient tests, so I'm especially interested in some help figuring out what else to test and how to go about it. I don't believe we need to test library behavior here because Clang does not provide any library support for complex types in freestanding mode; we expect the user to link against a C standard library that has such support.

https://github.com/llvm/llvm-project/pull/88161