[clang] [OpenACC] Implement 'var' parsing correctly, support array sections (PR #77617)

[Erich Keane via cfe-commits]

erichkeane wrote:

@alexey-bataev : In particular I'd like to make you aware of what I've done here, since this is similar to the work you've done before/will affect it.  It is currently my thought to make the OMPArraySectionExpr type a generic 'ArraySectionExpr' type in the not-so-distant future (PERHAPS with the OMP and OpenACC types inheriting from it?  But perhaps those types aren't necessary), so I anticipate you'll be seeing stuff like this in the future.

https://github.com/llvm/llvm-project/pull/77617