Implement constexpr std::vector

[miscco]

Fixes #45

NOTE: The first commit simply squashed the in flight PRs to simplify review

[miscco]

@CaseyCarter, @StephanTLavavej I seem to recall that I found those unexplicable queue errors in one of the earlier PRs

[miscco]

There were some merge conflicts with this and the previous PRs, so I went ahead and cleaned them up and dropped one.

[miscco]

There is more of them!
https://reviews.llvm.org/D68365

[StephanTLavavej]

Is this blocked on a Clang compiler bug that needs to be filed, or do you need assistance reducing the weird compiler error to a self-contained test case that can be filed?

[mnatsuhara]

🥳

[StephanTLavavej]

Pushed (and mirrored) two changes:

• yvals_core.h disables constexpr vector for Clang, but the test was defining input arrays guarded by MSVC_INTERNAL_TESTING (which is defined for all compilers internally), even though none of the following tests are active for Clang. It warns that the arrays are unused. The simplest fix is to suppress the warning, tagged with the LLVM bug number. When we can enable and test this feature for Clang, we can remove the warning suppression.
• There were a couple occurrences of this never-defined-in-GitHub macro that were _UGLY but it is a non-ugly macro (as only test code detects it, never product code). Fixed those comments.

[StephanTLavavej]

Thanks for constexprizing the STL's best container, and finding tons of ways to improve compiler quality! 😹 🚀 🎉

[miscco]

Huge shoutout to @mnatsuhara for her incredible work on all the compiler fixes 🎉