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

[Joseph Huber via cfe-commits]

jhuber6 wrote:

Test should probably show that IR is equivalent to `#pragma omp requires unified_shared_memory` or however that's spelled. Basic documentation should be provided by the help test in the new flag, but we probably have somewhere in the OpenMP docs you could add it to if desired.

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