[llvm] [openmp] [OpenMP] Fix runtimes default build (PR #149871)

[via llvm-commits]

jprotze wrote:

> `omp.h` is moved from `lib/clang/22/include/omp.h` to `include/omp.h`.

This fits to my in code comment above.
 
> We use standard 3-stage LTO-PGO-BOLT builds, which primarily use https://github.com/llvm/llvm-project/blob/main/clang/cmake/caches/BOLT-PGO.cmake. You can find top-level community doc at https://llvm.org/docs/AdvancedBuilds.html

These documents don't include OpenMP in their builds.
In the caches directors, only `VectorEngines.cmake` builds OpenMP via `LLVM_ENABLE_RUNTIMES`. 
The important question: do you build OpenMP via `LLVM_ENABLE_PROJECTS` or `LLVM_ENABLE_RUNTIMES`?

https://github.com/llvm/llvm-project/pull/149871