[clang] [CUDA][Win32] Add `fma(long double,..)` to math forward declares. (PR #73756)

[Reid Kleckner via cfe-commits]

rnk wrote:

> @rnk -- what's the best way to check for compilation with microsoft's stardard C++ library?

If Clang is compiling with the MSVC STL headers, it should be defining `_MSC_VER`, usually `-fms-compatibilty` has to be enabled to compile  MSVC STL headers. However, I searched, and it looks like the MSVC STL defines [this macro](https://github.com/microsoft/STL/blob/main/stl/inc/yvals_core.h#L883C9-L883C27), _MSVC_STL_VERSION . I don't know when they started defining that, but it has the right meaning.

https://github.com/llvm/llvm-project/pull/73756