[clang] [LifetimeSafety][NFC] Refactor OriginList to OriginNode tree (PR #194797)

[Gábor Horváth via cfe-commits]

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@@ -92,30 +92,42 @@ struct Origin {
 ///
 /// The list structure enables the analysis to track how loans flow through
 /// different levels of indirection when assignments and dereferences occur.
-class OriginList {
+///
+/// TODO: Currently list-shaped (each node has at most one pointee child).
+/// Will become tree-shaped once field children are added to support
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Xazax-hun wrote:

We might have trees as a result of processing named lifetime annotations in the future. 

https://github.com/llvm/llvm-project/pull/194797