variant
This is a follow-up and depends on #139368 being merged first. Implements [P2944R3](https://wg21.link/P2944R3) partially, which adds constrained comparisons to `std::variant` Closes #136769 Depends on #139368 # References [variant.relops](https://wg21.link/variant.relops)
We don't neeed to handle both spellings anymore since we don't support Clang 15 anymore.
I made sure they all had some expected-error output in them. Many of these tests would be better implemented as a positive test using SFINAE, but this is beyond the scope of this patch. Differential Revision: https://reviews.llvm.org/D153980