Update variant "all types same/convertible" metaprogramming

[CaseyCarter]

And a few other minor changes while we're here. I've separated changes out by commit for ease of review, but this should be squash merged. The most significant change here - others are self-explanatory - replaces the metaprogramming used to enforce the Standard's requirements that "For each valid pack m, e(m) is a valid expression. All such expressions are of the same type
and value category" ([variant.visit]/5).

here are different mechanisms to validate the requirement for visit<R> and for plain visit. Previously, we created a single typelist where each type represented one possible configuration of the set of argument variants to a visit call, and then folded over a single giant pack expansion of visitor evaluations ensuring they had the same type and value category (or are all convertible to R for visit<R>). This was enormously expensive for throughput when the number of possible states is large since the compilers burn a lot of memory maintaining long typelists. The new version uses tree evaluation, where each internal node of the tree ands together the result of evaluating its child nodes, which each correspond to a possible state of a single argument variant. Leaves correspond to potential calls and have a boolean value indicating whether that potential call has the same type and value category as the first valid configuration (has a type and value category convertible to the result type for visit<R>).

The resulting speedup for the long-pole /analyze configs of the visit tests in the "Update LLVM" PR is around 50%.

Fixes #2770.
Unblocks #2976.

[StephanTLavavej]

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

[StephanTLavavej]

Thanks for making variant faster to compile! 🚀 🐆 🐇