ASTVerifier: make sure isFinal and FinalAttr are in sync (#26740)

Serialization is currently working around them /not/ having been in
sync in the past, but they are now, and we might as well start
depending on it.

Author: jrose-apple

lib/AST/ASTVerifier.cpp

@@ -2362,6 +2362,14 @@ class Verifier : public ASTWalker {
         }
       }
 
+      if (VD->isFinal() != VD->getAttrs().hasAttribute<FinalAttr>()) {
+        Out << "decl should be final iff it has FinalAttr, but isFinal() = "
+            << VD->isFinal() << " and hasAttribute<FinalAttr>() = "
+            << VD->getAttrs().hasAttribute<FinalAttr>() << "\n";
+        VD->dump(Out);
+        abort();
+      }
+
       verifyCheckedBase(VD);
     }