[clang] [llvm] [OpenMP] Add directive spellings introduced in spec v6.0 (PR #141772)

[Alexey Bataev via cfe-commits]

alexey-bataev wrote:

> @alexey-bataev, could you clarify what tests you had in mind?

Would be good to see the new test for the new spellings, like `omp parallel_for`

https://github.com/llvm/llvm-project/pull/141772