[clang] [BoundsSafety][Clang][Sema][AST] Introduce CThisExpr for C __counted_by attributes (PR #199241)

[Oliver Hunt via cfe-commits]

https://github.com/ojhunt requested changes to this pull request.

A bunch of comments.

You're adding a new AST node so you should add serialization and deserialization tests as well

https://github.com/llvm/llvm-project/pull/199241