[clang] [OpenMP] Allow OMP6.0 features. (PR #122108)

[Zahira Ammarguellat via cfe-commits]

zahiraam wrote:

> Please address @saiislam's questions. I also think that the title and description of the PR should reflect that this PR not only introduces the 6.0 flag, but also moves several of the tests to test against 5.1 and 6.0.

I responded to the question.
The changes in the tests are against 6.0 only. I have only added run lines with `fopenmp=6.0`.

https://github.com/llvm/llvm-project/pull/122108