[llvm] [openmp] Deprecate LLVM_ENABLE_PROJECTS in favor of LLVM_ENABLE_RUNTIMES (PR #124711)

[Michael Kruse via llvm-commits]

Meinersbur wrote:

> > Flang currently only works with `LLVM_ENABLE_PROJECTS=openmp`:
> 
> I'm afraid this is not true in the first place. My nightly tests builds openmp as runtime and I'm using flang+openmp to test my applications.

Where do you get `omp_lib.mod` from?

https://github.com/llvm/llvm-project/pull/124711