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

[Zahira Ammarguellat via cfe-commits]

zahiraam wrote:

> I don't think it is necessary to test each value of `-fopenmp-version=` separately, but don't mind either as long it doesn't affect maintanability.
> 
> * Instead of repeating each clang diagnositic, consider passing multiple values to `-verify`, e.g. `-verify=omp51,omp60`, `-verify=omp51-exact,omp60-or-lafter`
> * Write expected diagnostics on separate lines using `@<relative line>`-syntax

I think this mostly applies to `declare_target_messages.cpp` and `distribute_simd_misc_messages.c` right? Should I change all the lines to the form `@<relative line>`-syntax?

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