[clang][AIX] Remove test for the default OpenMP runtime

The default OpenMP runtime may not be libomp since it can be changed
by specified `CLANG_DEFAULT_OPENMP_RUNTIME`. This test will fail if
we change the default OpenMP runtime.

This patch removes test for the default OpenMP runtime and moves the
CHECKs downward.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D143549