Revert "[OpenMP][FIX] Restrict more unsound assmptions about threading"

This reverts commit 07c37534.

Reason: This change is dependent on a commit that needs to be rolled
back because it broke the ASan buildbot. See
https://reviews.llvm.org/rGfc21f2d7bae2e0be630470cc7ca9323ed5859892 for
more information.