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

[via cfe-commits]

h-vetinari wrote:

I'm surprised about the "compiler-rt is not supported on windows" comment - in our distribution, we've been building compiler-rt on windows for years, and I don't remember significant issues.

 It's true that `__divsc3` is not available _without_ compiler-rt (i.e. just relying on the msvc runtime libraries), but together with compiler-rt these things are generally possible? Not that a similar [situation](https://github.com/llvm/llvm-project/issues/54596) exists on `osx-arm` for example - in certain situations, linking `compiler-rt` is unavoidable.

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