[clang] [analyzer] Avoid unnecessary super region invalidation in `CStringChecker` (PR #146212)

[Donát Nagy via cfe-commits]

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

LGTM, thanks for the updates!

Before merging the commit consider adding a note to the PR description / commit message that in the case of `strlcat` you use `size` as an approximation of the exact `amountCopied`.

https://github.com/llvm/llvm-project/pull/146212