[clang][AST] fix lack comparison of declRefExpr in ASTStructuralEquivalence (#66041)

Fixed #66047
Before fix,the following testcase expected true.
```cpp
TEST_F(StructuralEquivalenceStmtTest, DeclRefENoEq) {
  std::string Prefix = "enum Test { AAA, BBB };";
  auto t = makeStmts(
      Prefix + "void foo(int i) {if (i > 0) {i = AAA;} else {i = BBB;}}",
      Prefix + "void foo(int i) {if (i > 0) {i = BBB;} else {i = AAA;}}",
      Lang_CXX03, ifStmt());
  EXPECT_FALSE(testStructuralMatch(t)); // EXPECT_TRUE
}
```

clang/docs/ReleaseNotes.rst

@@ -244,6 +244,8 @@ Bug Fixes in This Version
   (`#65156 <https://github.com/llvm/llvm-project/issues/65156>`_)
 - Clang no longer considers the loss of ``__unaligned`` qualifier from objects as
   an invalid conversion during method function overload resolution.
+- Fix lack of comparison of declRefExpr in ASTStructuralEquivalence
+  (`#66047 <https://github.com/llvm/llvm-project/issues/66047>`_)
 - Fix parser crash when dealing with ill-formed objective C++ header code. Fixes
   (`#64836 <https://github.com/llvm/llvm-project/issues/64836>`_)
 - Clang now allows an ``_Atomic`` qualified integer in a switch statement. Fixes

clang/lib/AST/ASTStructuralEquivalence.cpp

@@ -214,6 +214,15 @@ class StmtComparer {
     return E1->size() == E2->size();
   }
 
+  bool IsStmtEquivalent(const DeclRefExpr *DRE1, const DeclRefExpr *DRE2) {
+    const ValueDecl *Decl1 = DRE1->getDecl();
+    const ValueDecl *Decl2 = DRE2->getDecl();
+    if (!Decl1 || !Decl2)
+      return false;
+    return IsStructurallyEquivalent(Context, const_cast<ValueDecl *>(Decl1),
+                                    const_cast<ValueDecl *>(Decl2));
+  }
+
   bool IsStmtEquivalent(const DependentScopeDeclRefExpr *DE1,
                         const DependentScopeDeclRefExpr *DE2) {
     if (!IsStructurallyEquivalent(Context, DE1->getDeclName(),

clang/unittests/AST/StructuralEquivalenceTest.cpp

@@ -2393,5 +2393,14 @@ TEST_F(StructuralEquivalenceCacheTest, GotoStmtNoEq) {
   EXPECT_FALSE(testStructuralMatch(S));
 }
 
+TEST_F(StructuralEquivalenceStmtTest, DeclRefExpr) {
+  std::string Prefix = "enum Test { AAA, BBB };";
+  auto t = makeStmts(
+      Prefix + "void foo(int i) {if (i > 0) {i = AAA;} else {i = BBB;}}",
+      Prefix + "void foo(int i) {if (i > 0) {i = BBB;} else {i = AAA;}}",
+      Lang_CXX03, ifStmt());
+  EXPECT_FALSE(testStructuralMatch(t));
+}
+
 } // end namespace ast_matchers
 } // end namespace clang