: Fix the default constructor of error_category conditionally

[A. Jiang (frederick-vs-ja)]

Fixes #3119.

Addresses #1116 (fixes it except for /clr:pure) and unblocks one libcxx test.

This PR would complicate Vinnie Falco (@vinniefalco)'s #3044 if it gets merged.

[nicole mazzuca (strega-nil-ms)]

I don't love that we have to do this to keep ABI, but this is certainly better than the alternative where this isn't constexpr.

[Stephan T. Lavavej (StephanTLavavej)]

Thanks, this is awesome! 😻 I think we can regard #1116 as completely fixed by this (/clr:pure has virtually no users, and there's nothing more we can do here, so no need to keep a bug open; everything mentioning /clr:pure will inherently be eradicated during vNext).

I pushed a conflict-free and no-reformatting-needed merge with main, followed by a couple of minor cleanups for /clr:pure, and a small enhancement adding noexcept to the _Addr_storage ctors so the compiler won't emit EH enforcement in debug mode (as the ctors that use this are marked noexcept). FYI nicole mazzuca (@strega-nil-ms) as you previously approved.

[Stephan T. Lavavej (StephanTLavavej)]

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

[Stephan T. Lavavej (StephanTLavavej)]

I've pushed an additional commit to silence a warning found by internal testing.

[Stephan T. Lavavej (StephanTLavavej)]

Thanks for fixing this bug we thought was unfixable! 🐞 🛠️ 😻