[clang] [OpenMP] Change default version to OMP6.0. (PR #122108)

[Zahira Ammarguellat via cfe-commits]

zahiraam wrote:

@alexey-bataev Can you please take a look at this draft PR? I think there are still some LIT tests failing but I want to make sure the changes I am proposing are correct before making further changes. I am not sure if the macro `_OPENMP` should be set to `202411` or if it should continue being equal to `202011`.

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