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

[Joseph Huber via llvm-commits]

jhuber6 wrote:

> This is a bad idea. Why do you want to remove that dependency so bad?

Because I want to put this back in `openmp/` and it'd be the only dependency on LLVM. 

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