[clang] [LifetimeSafety] Propogate loans through chained assignment (PR #198510)

[Gábor Horváth via cfe-commits]

https://github.com/Xazax-hun approved this pull request.

Overall, looks good to me. But I wonder if this should be split up in 2 PRs. 

https://github.com/llvm/llvm-project/pull/198510