[libc++][NFC] Rename __libcpp_*_or_builtin to __constexpr_*

We normally use the `__libcpp_` prefix to backport functions, but that doesn't really make sense here. The functions are always available. Using `__constexpr_` makes the intended use of the functions clear.

Reviewed By: Mordante, #libc

Spies: libcxx-commits

Differential Revision: https://reviews.llvm.org/D139521