[clang] [OpenMP][USM] Adds test for -fopenmp-force-usm flag (PR #75467)

[Jan Patrick Lehr via cfe-commits]

jplehr wrote:

I updated the feature PR (#75468) with a different solution. Will update the test after feedback if the route I took in the other PR is seen as OK.

https://github.com/llvm/llvm-project/pull/75467