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

[Shilei Tian via llvm-commits]

shiltian wrote:

That's not going to completely work, right? If someone builds `openmp/` out-of-tree using GCC as the compiler and sets `CMAKE_CXX_COMPILER_TARGET` to `amdgcn`, it's going to break.

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