[clang] [clang][analyzer] Improve PointerSubChecker (PR #96501)

[Donát Nagy via cfe-commits]

https://github.com/NagyDonat approved this pull request.

LGTM.

I'm a bit surprised to see that this checker duplicates the logic of the array bounds checkers (in the special case when the indexing operation is within a pointer subtraction). Right now this is OK but we'll need to delete this once ArrayBoundV2 becomes stable.

https://github.com/llvm/llvm-project/pull/96501