[libc++abi] NFC: Move AtomicInt to cxa_guard_impl.h

Since the atomic_support.h header of libc++abi is considered technical
debt (since we should use libc++'s), it's better not to add new
definitions to it, which makes it diverge from the original libc++
header even more.

Differential Revision: https://reviews.llvm.org/D75950