[openmp] Allow testing OpenMP without a full clang build tree (#182470)

Having a build tree with "not" and "FileCheck" is still required, but if
Clang/Flang isn't configured in that build, run the tests with the same
compiler CMake uses. This is how testing worked in the standalone build
configurations that now have been removed.

GitOrigin-RevId: 48a5119d8e7d7236a28c14d06ec215ef3366ef90