[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
}
```