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

[Johannes Doerfert via cfe-commits]

jdoerfert wrote:

I mean, the test shows device IR for two functions, doesn't it? I am not aware that part is impacted by USM. Host globals/register functions are.

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