<stdatomic.h>: Fix preprocessor logic

[barcharcraz]

Because this header needs to work in C and C++, we needed to move the check for a non-broken preprocessor outside of yvals_core, but we didn't actually skip the whole header if it failed!

This PR fixes that. In addition, it:

• Reverses the conditional logic to avoid needing to use an "else" branch
• Changes the #if _STL_COMPILER_PREPROCESSOR to _STL_INTERNAL_STATIC_ASSERT(_STL_COMPILER_PREPROCESSOR);

[StephanTLavavej]

Relabeling - I don't think that this was a bug, strictly speaking, as for non-compiler tools we'd skip the C check, include yvals.h (which would skip its contents), and then skip the rest of the file - which should work (if my vague understanding of the non-compiler tools is correct).

I do believe that simply skipping the whole thing is an improvement.

[barcharcraz]

Relabeling - I don't think that this was a bug, strictly speaking, as for non-compiler tools we'd skip the C check, include yvals.h (which would skip its contents), and then skip the rest of the file - which should work (if my vague understanding of the non-compiler tools is correct).

I do believe that simply skipping the whole thing is an improvement.

Good point, the comment was confusing though and it caused me to write my own bug when modifying the header, so I would also say it's an improvement

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[StephanTLavavej]

Thanks for simplifying this code! ✨ 😸 ✅