These are the last™ changes to the tests for constexpr preparation.
Reviewed By: Quuxplusone, #libc, Mordante
Spies: Mordante, EricWF, libcxx-commits
Differential Revision: https://reviews.llvm.org/D120951
After this patch, we can use `--param std=c++20` even if the compiler only
supports -std=c++2a. The test suite will handle that for us. The only Lit
feature that isn't fully baked will always be the "in development" one,
since we don't know exactly what year the standard will be ratified in.
This is another take on https://reviews.llvm.org/D99789.
Differential Revision: https://reviews.llvm.org/D100210