[clang] [flang] [flang][OpenMP] Bump default OpenMP version to 3.1 (PR #133745)

[Tarun Prabhu via cfe-commits]

https://github.com/tarunprabhu approved this pull request.

Thanks Tom. This looks fine to me, but please wait for those more familiar with OpenMP before merging.

https://github.com/llvm/llvm-project/pull/133745