[Support] Do not run test on AIX

A part of the unit test CommandLineTest/BadResponseFile, added in
the commit fd3d7a9f (Handle errors in expansion of response files)
fails on AIX. The test checks if an error is issued when `file` in
`@file` is a path to directory. This change excludes that check on
AIX platform.

Differential Revision: https://reviews.llvm.org/D136090