Prevent dependancy on libatomic when using GCC to provide <atomic>.

The __atomic_is_lock_free(...) function sometimes requires linkage to libatomic
if it cannot be evaluated at compile time. Remove __c11_atomic_is_lock_free
and use __atomic_is_lock_free(sizeof(Tp)) directly so that it can be evaluated
at compile time.

llvm-svn: 239648