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

[Jan Patrick Lehr via cfe-commits]

jplehr wrote:

The IR is impacted for the global that is in that test case. Lines ~46-50 (first IR section) vs line ~68 (second IR section). The remaining code is indeed the same.
The way that this test is executed is out of date however, given that I have reimplemented the flag.
My plan is to keep this test and add a few runtime tests as well, since we should be able to validate that we do not see data transfers.

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