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

[Erich Keane via cfe-commits]

erichkeane wrote:

Note: this is one of my approaches.  This ends up touching the OpenMP work 'the least' (in that this is the entirety of the OpenMP changes as best I can tell), at the expense of more work/a separate AST Node for OpenACC.  

The OTHER approach I have is to move/rename `OpenMPArraySectionExpr` to just `ArraySectionExpr`, and give it an `IsOpenACC` or `IsOpenMP` type function, and change everywhere it is used to check/assert on that.  

I'm totally open to other ideas though, I'd love to have a good solution for this.

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