Implement SSO for constexpr string

[miscco]

In #1502 we found that it should be possible to enable SSO, but we wanted to get constexpr string in.

It turns out that we were not that far off. I verified that all test compile fine with clang-12. That said, it seems I uncovered 2 distinct MSVC bugs:

1. The test for basic_string::operator-> is failing. MSVC complains about an uninitialized symbol. Clang is happy. I have no idea what is wrong there.
2. Clang complained heavily about switching back to SSO form. It turns out that previously we were directly constructing the sentinel in the first slot of the SSO array. However, we never switched from the _Ptr to the _Buf member. So I believe clang is correct here and MSVC should recognize that _Bufs lifetime did not yet start

[mnatsuhara]

Thanks for going through and working on this! Sorry for the wait on a review here. Things look pretty good to me, just a few questions / nitpicks and request for more extensive test coverage :)

[mnatsuhara]

The only real "request changes" here is the missing _STD, otherwise looks great! Thanks for all of the bug fixes and extensive added test coverage 😻

[StephanTLavavej]

Thanks, this is so much easier to understand now! 😻

I pushed changes to remove one more workaround (didn't affect correctness).

[CaseyCarter]

I'm going to push a tiny change to address the pre-existing issue and approve.

[StephanTLavavej]

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

[StephanTLavavej]

Thanks for simplifying these codepaths and making basic_string easier to maintain! 🎉 ✅ 😺