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

[Joseph Huber via llvm-commits]

jhuber6 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?

It's not an issue conceptually, we could definitely make it work, but there's no reason to make `openmp/` depend on LLVM headers just for a single constant `1` value. That's a lot of searching for user paths and setting up include directories that we don't need to do. That's separate from the compiler. This is not solving a fundamental issue, it's just solving the fact that I don't want to need to copy a million lines of CMake back into `openmp` just to find a single header that does `#define 1` basically.

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