[AIX] Default AIX to using -fno-use-cxa-atexit

On AIX, we use __atexit to register dtor functions rather than __cxa_atexit.
So a driver change is needed to default AIX to using -fno-use-cxa-atexit.

Windows platform does not uses __cxa_atexit either. Following its precedent,
we remove the assertion for when -fuse-cxa-atexit is specified by the user,
do not produce a message and silently default to -fno-use-cxa-atexit behavior.

Differential Revision: https://reviews.llvm.org/D82136