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

[A. Jiang via cfe-commits]

frederick-vs-ja wrote:

> But fails on Windows.

This seems because of that MS UCRT's complex math functions are non-conforming.

I _guess_ they could work if we write the following before including UCRT's `<complex.h>`.
```C
#define _C_COMPLEX_T
typedef double _Complex _C_double_complex;
typedef float _Complex _C_float_complex;
typedef long double _Complex _C_ldouble_complex;
```

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