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

[Alexey Bataev via cfe-commits]

alexey-bataev wrote:

Not sure this level of support is enough to make 6.0 the default version. We're missing support for some 5.2 features still

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