[clang] [Clang] Extend lifetime bound analysis to support assignments (PR #96475)

[Ilya Biryukov via cfe-commits]

ilya-biryukov wrote:

@hokein could you please split this into two PRs, one with an NFC change and another one with adding assignment support?
GitHub is not great at providing the UI to review individual commits and we won't be able to merge as 2 separate commits with the green button (which seems like a desirable end state for the readability of the commit history).

https://github.com/llvm/llvm-project/pull/96475