Add noexcept throughout <atomic>

Add noexcept throughout <atomic>

The CMake CheckLibcxxAtomic module was always failing to compile
the example, even when libatomic wasn't needed. This was caused
because the check doesn't link a C++ runtime library to provide
std::terminate, which is required for exception support.

The check is still really broken, but <atomic> is better!

llvm-svn: 364146