[clang] [LifetimeSafety] Implement multi-level origins (PR #168344)

[Gábor Horváth via cfe-commits]

https://github.com/Xazax-hun commented:

I just did a very rough first pass, will need to take a second look. I wonder if it is a good idea to call it an `OriginTree` when it is actually a list. I understand that it might become a tree later once/if we have named lifetimes, but I wonder if it is a bit confusing to call it a tree in the interim period. 

https://github.com/llvm/llvm-project/pull/168344