[libcxx] Remove _LIBCPP_HAS_NO_BUILTIN_IS_CONSTANT_EVALUATED

All supported compilers should support
_LIBCPP_HAS_NO_BUILTIN_IS_CONSTANT_EVALUATED so this can be removed.

Reviewed By: ldionne, #libc, Quuxplusone

Differential Revision: https://reviews.llvm.org/D107239