[clang] [analyzer] Improve handling of placement new in `PointerArith` (PR #155855)

[Alejandro Álvarez Ayllón via cfe-commits]

alejandro-alvarez-sonarsource wrote:

Sorry for the delay in answering, I was waiting for @steakhal to be back to double check with him, but he already answered with what I had anyways in mind.
Indeed it is a one-off fix. I may revisit the checker soon-ish, but I have no long-term plans besides incremental changes until I see I have to.

https://github.com/llvm/llvm-project/pull/155855