Disable use of _ExtInt with '__atomic' builtins

We're (temporarily) disabling ExtInt for the '__atomic' builtins so we can better design their behavior later. The idea is until we do an audit/design for the way atomic builtins are supposed to work with _ExtInt, we should leave them restricted so they don't limit our future options, such as by binding us to a sub-optimal implementation via ABI.

Example after this change:

    $ cat test.c

        void f(_ExtInt(64) *ptr) {
          __atomic_fetch_add(ptr, 1, 0);
        }

    $ clang -c test.c

        test.c:2:22: error: argument to atomic builtin of type '_ExtInt' is not supported
          __atomic_fetch_add(ptr, 1, 0);
                             ^
        1 error generated.

Differential Revision: https://reviews.llvm.org/D84049