Fix crash in AST printing

Extensions need not have nominal types, e.g.

    extension Int : Codable { ... }

where

    typealias Codable = Encodable & Decodable

Author: Itai Ferber

lib/AST/ASTPrinter.cpp

@@ -203,7 +203,8 @@ struct SynthesizedExtensionAnalyzer::Implementation {
   unsigned countInherits(ExtensionDecl *ED) {
     unsigned Count = 0;
     for (auto TL : ED->getInherited()) {
-      if (shouldPrint(TL.getType()->getAnyNominal(), Options))
+      auto *nominal = TL.getType()->getAnyNominal();
+      if (nominal && shouldPrint(nominal, Options))
         Count ++;
     }
     return Count;