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

[Zahira Ammarguellat via cfe-commits]

zahiraam wrote:

> > > I'm not convinced I have sufficient tests, so I'm especially interested in some help figuring out what else to test and how to go about it. I don't believe we need to test library behavior here because Clang does not provide any library support for complex types in freestanding mode; we expect the user to link against a C standard library that has such support.
> > 
> > 
> > I haven't looked at it in details yet, but the test needs to run on Linux and Windows.
> 
> I don't think there's anything platform-specific about our support at this level, so the test has no triples in the RUN line (so it will be run on all triples we test on which includes Linux and Windows). Or do you mean something else?

I guess at the IR level no. It's just that I have recently found that this test:
`#include <stdio.h>`
`#include <complex.h>`

`int main()`
`{`
    `float complex b = -8.45345913e+29f + -1.23588801e+31f * I;`
   ` float complex a = 1.25284138e+31f + -7.91789592e+30f * I;`
    `float complex c = a / b;`
    `printf("%f %f\n", creal(c), cimag(c));`
    `return 0;`
`}`
But fails on Windows.  

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