[clang] [NFC] Factor out common parts of ArraySections into its own class (PR #89639)

[Erich Keane via cfe-commits]

erichkeane wrote:

> > @alexey-bataev Not having this new 'base' type as a member of the `StmtNodes.td` results in one of the Clang tools failing (see the CI). AND, since it is a template, it can't do that.
> > I can't make it a Mixin, because it needs to provide some of the Expr-type functions regarding source location/etc.
> > So I don't have a great idea on how to have reuse between the two while still having them be separate AST nodes (as there IS a bit of a difference in behaviors).
> > I COULD just move the OMP type to Expr.h (renamed as just ArraySectionExpr), add a flag for OMP vs OpenACC, then make the logic of each function check it, but I'm tentative as to whether that is better.
> 
> I'm fine with having just a single AST node

Ok, I'll start on the work to make this happen.  

https://github.com/llvm/llvm-project/pull/89639