[llvm] [OpenMP] Remove dependency on LLVM include directory from DeviceRTL (PR #136359)

[Shilei Tian via llvm-commits]

shiltian wrote:

In the past if I remember correctly, we need more values from LLVM headers for AMDGPU, such as the wavefront size. It seems like as we evolve recently, we only need one value. It might be "overkill" to include it.

If we move it back to `openmp/`, it will not be built by default right? A lot of `libomp` users build `libomp` out of tree. With that being said, even if we move it back, we can probably still have it depend on LLVM headers if the device runtime is built?

https://github.com/llvm/llvm-project/pull/136359