[clang] [llvm] [OpenMP] Adds omp_target_is_accessible routine (PR #138294)

[via llvm-commits]

nicebert wrote:

after Monday's discussion in the Accelerator subcommittee call I'm re-working the implementation to what was discussed & create a pr to clarify the wording in the spec.

https://github.com/llvm/llvm-project/pull/138294