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

[Shilei Tian via llvm-commits]

shiltian wrote:

> We already require that the device RTL is built with a 'just-built' clang that we pull out of the user's path.

If that is the case, to depend on LLVM headers is not an issue?

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