[clang] [LifetimeSafety] Use ImmutableList for buildOriginFlowChain DFS nodes (PR #208890)

[Utkarsh Saxena via cfe-commits]

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@@ -668,10 +668,10 @@ class LifetimeSafetySemaHelperImpl : public LifetimeSafetySemaHelper {
     if (OriginExprChain.empty())
       return;
 
-    const Expr *LastExpr = OriginExprChain.back();
+    const Expr *LastExpr = OriginExprChain.front();
     std::string IssueStr = getDiagSubjectDescription(LastExpr);
 
-    for (const Expr *CurrExpr : reverse(OriginExprChain.drop_back())) {
+    for (const Expr *CurrExpr : OriginExprChain.drop_front()) {
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usx95 wrote:

Just `llvm::reverse(
            llvm::SmallVector<OriginID>(CurrNode.OriginFlowChain.begin(),
                                        CurrNode.OriginFlowChain.end()))` should be fine I think.

https://github.com/llvm/llvm-project/pull/208890