AST: Fix some warnings

lib/AST/DeclContext.cpp

@@ -511,7 +511,7 @@ ResilienceExpansion DeclContext::getResilienceExpansion() const {
   for (const auto *dc = this; dc->isLocalContext(); dc = dc->getParent()) {
     // Default argument initializer contexts have their resilience expansion
     // set when they're type checked.
-    if (auto *DAI = dyn_cast<DefaultArgumentInitializer>(dc)) {
+    if (isa<DefaultArgumentInitializer>(dc)) {
       return cast<AbstractFunctionDecl>(dc->getParent())
           ->getDefaultArgumentResilienceExpansion();
     }