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

[Nikolas Klauser via cfe-commits]

philnik777 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.

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