[libc++] Fix diagnostic for <stdatomic.h> before C++23 (#83351)

We normally try to issue a reasonable diagnostic when mixing
<stdatomic.h> and <atomic> before C++23. However, after granularizing
the <atomic> header, the check and the #error message was moved to
*after* the point where mixing both causes problems. When mixing both
headers, we would hence get the diagnostic burried under a pile of
previous diagnostics in e.g. __atomic/kill_dependency.h.

This patch moves the check earlier to restore the intended behavior. It
also switches from `#ifdef kill_dependency` to an explicit check of the
inclusion of the header and the Standard version, which seems to be more
reliable than checking whether a macro is defined.