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

[Gábor Horváth via cfe-commits]

================
@@ -64,29 +93,48 @@ void FactsGenerator::run() {
 void FactsGenerator::VisitDeclStmt(const DeclStmt *DS) {
   for (const Decl *D : DS->decls())
     if (const auto *VD = dyn_cast<VarDecl>(D))
-      if (hasOrigin(VD))
-        if (const Expr *InitExpr = VD->getInit())
-          killAndFlowOrigin(*VD, *InitExpr);
+      if (const Expr *InitExpr = VD->getInit()) {
+        OriginTree *VDTree = getTree(*VD);
+        if (!VDTree)
+          continue;
+        OriginTree *InitTree = getTree(*InitExpr);
+        assert(InitTree && "VarDecl had origins but InitExpr did not");
+        // Special handling for rvalue references initialized with xvalues.
----------------
Xazax-hun wrote:

What about regular (non-rvalue) references? I am not sure I understand why is this case more special than that. 

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