[Diagnostics] Use ´isa instead of dyn_cast` because array/dictionary expr in unused

Author: xedin

lib/Sema/CSDiagnostics.cpp

@@ -671,10 +671,11 @@ bool GenericArgumentsMismatchFailure::diagnoseAsError() {
     }
 
     case ConstraintLocator::TupleElement: {
-      if (auto *array = dyn_cast<ArrayExpr>(getRawAnchor()))
-        diagnostic = getDiagnosticFor(CTP_ArrayElement);
+      auto *anchor = getRawAnchor();
 
-      if (auto *dict = dyn_cast<DictionaryExpr>(getRawAnchor())) {
+      if (isa<ArrayExpr>(anchor)) {
+        diagnostic = getDiagnosticFor(CTP_ArrayElement);
+      } else if (isa<DictionaryExpr>(anchor)) {
         auto eltLoc = last.castTo<LocatorPathElt::TupleElement>();
         diagnostic = getDiagnosticFor(
             eltLoc.getIndex() == 0 ? CTP_DictionaryKey : CTP_DictionaryValue);