Fix PR24114 - std::atomic for non-Clang is not a literal type

Add _LIBCPP_CONSTEXPR to the implementation of __gcc_atomic_t.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@242172 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed