[clang] [analyzer] Adjust ExprEngine::VisitDeclStmt to ignore `T v=v;` idiom cases for C code (PR #187530)

[Donát Nagy via cfe-commits]

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@@ -624,6 +624,20 @@ void ExprEngine::VisitDeclStmt(const DeclStmt *DS, ExplodedNode *Pred,
     return;
   }
 
+  // Bypass a nop initialization that assign to itself at variable declaration.
+  // I.e., int x = x;
+  // This is an idiom in C code and GCC will not generate any assemblies for
+  // this self initialization, even under -O0, but Clang will.
+  // Since the frontend will warn in C++ code, and it is ill-formed for C++
+  // reference types, the bypass is effected to C code only.
+  if (getContext().getLangOpts().getCLangStd())
+    if (const Expr *EI = VD->getInit())
+      if (const DeclRefExpr *DR = dyn_cast<DeclRefExpr>(EI->IgnoreImpCasts()))
+        if (VD == DR->getDecl()) {
+          Dst.insert(Pred);
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NagyDonat wrote:

Actually, never mind this suggestion – I thought about this and now I'm leaning towards renaming `ExplodedNodeSet::Add` to `insert`. 

https://github.com/llvm/llvm-project/pull/187530