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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> Note that
> 
> ```c++
> auto div(_Complex float lhs, _Complex float rhs) {
>   return lhs / rhs;
> }
> 
> int main() {
>   return __real div(1.f, 2.f);
> }
> ```
> 
> will fail to link on windows due to `__divsc3` not being available. Similar programs probably also fail due to other compiler-rt functions not being available.

I *think* Clang still gets to claim conformance here because we handle the language side of things correctly and the user is responsible for linking to a runtime library with appropriate support for the platform. However, this sure does straddle the line in some ways, so I could see this being a reason to claim "partial" conformance.

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