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

[Erich Keane via cfe-commits]

erichkeane wrote:

@njames93 : My test failure has to do with the "NodeIntrospection" stuff that you seemed to review ~3 years ago: https://reviews.llvm.org/D93164

At the moment, I cannot even figure out how to get it to build, but I suspect it is getting confused in some way by having a non-AST-node base class of a type?  Any help you could give would be appreciated.

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