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

[Joseph Huber via llvm-commits]

jhuber6 wrote:

> Flang currently only works with `LLVM_ENABLE_PROJECTS=openmp`:
> 
> https://github.com/llvm/llvm-project/blob/6d0dd3d5c41e5b564714de9226c0623012538051/flang/tools/f18/CMakeLists.txt#L144-L164
> 
> Cannot remove it before Flang also supports LLVM_ENABLE_RUNTIMES=openmp

Why does `flang` depend on OpenMP? It's not like `clang` does.

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