[clang] [OpenMP][USM] Introduces -fopenmp-force-usm flag (PR #76571)

[Jan Patrick Lehr via cfe-commits]

jplehr wrote:

> > Is the approach taken in this approach acceptable as opposed to the header solution I put up earlier?
> 
> Yes, it's pretty much exactly what I had in mind from my suggestion in the last PR. Thanks.

Perfect. I'll go ahead and add lit and runtime tests.

https://github.com/llvm/llvm-project/pull/76571