Add std::is_constant_evaluated.

Clang recently added __builtin_is_constant_evaluated() and GCC 9.0
has it as well.

This patch adds support for it in libc++.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@359119 91177308-0d34-0410-b5e6-96231b3b80d8
10 files changed