_STD-qualify _Ugly non-member function calls in <variant>

[CaseyCarter]

Qualifies calls to namespace-scope functions in <variant>, and enables the libc++ ADL test that now passes.

Partially addresses #140

[StephanTLavavej]

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

[StephanTLavavej]

I've had to push an additional commit to fix internal test failures with /clr. I was a bad kitty and didn't investigate the root cause (or mark this as TRANSITION), but it appears that /clr is deciding to instantiate stuff at the end of the TU, and this ends up requiring completeness. Since this is a fairly niche use case, I figured that just skipping the affected portion for /clr would be strictly better than the status quo where it's unconditionally skipped.

[CaseyCarter]

I was a bad kitty and didn't investigate the root cause (or mark this as TRANSITION)

I'm the last person you'll have to convince that we should do any extra work where /clr is concerned.

[StephanTLavavej]

Thanks for investigating and fixing this! 🕵️ 🛠️ 🎉