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

[Nikolas Klauser via cfe-commits]

https://github.com/philnik777 approved this pull request.

LGTM from my side. I'll leave the conformance test details to the folks who know what they're doing.

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