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

[Jan Patrick Lehr via cfe-commits]

jplehr wrote:

I just realized that I need to update the clang lit tests, so this is *not ready to land*, but I don't see a button to indicate that.

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