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

[Joseph Huber via llvm-commits]

jhuber6 wrote:

> How are you gonna compile the entire `openmp/` project with the GPU triple?

```
if (${TRIPLE} MATCHES("^nvptx64|^amdgcn"))
  add_subdirectory(device)
else()
  add_subdirectory(runtime)
endif()
```

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