[clang] [BoundsSafety] WIP: Make 'counted_by' work for pointer fields; late parsing for 'counted_by' on decl attr position (PR #87596)

[Dan Liew via cfe-commits]

delcypher wrote:

@rapidsna @bwendling I've put the experimental feature flag for late parsing in a separate PR (#88596). I've not updated this PR to be based on that PR yet but will do soon. I figured the code for experimental feature flag was large enough that it deserved its own PR to make things easier to review.

https://github.com/llvm/llvm-project/pull/87596