[clang] [Clang][OpenMP 6.0] Support positional arguments in `adjust_args` clause (PR #225081)

[via cfe-commits]

ykhatav wrote:

> > > Do not quite understand why you need new expressions?
> > 
> > 
> > omp_num_args and ranges could be represented by plain structure containing the item kind and its bound expressions but we would need to handle them explicitly for serialization, traversal,printing, and template substitution. Storing them as expression nodes lets them participate in the existing AST machinery once the required node hooks are added. Since OMPDeclareVariantAttr currently stores adjust_args items as Expr *, this seemed like the natural representation. What representation would you prefer?
> 
> I would not add a new expressions for these, better to have a clause-like stuff, if possible

Done.

https://github.com/llvm/llvm-project/pull/225081