Don't check the inheritance clauses of extensions as part of validating the type they extend.

This makes the type checking slightly lazier for declarations, but is
otherwise NFC.

Swift SVN r20686

lib/AST/Module.cpp

@@ -638,6 +638,9 @@ findExplicitConformance(NominalTypeDecl *nominal, ProtocolDecl *protocol,
 
       // Visit the extensions of this type.
       for (auto ext : currentNominal->getExtensions()) {
+        if (resolver)
+          resolver->resolveExtension(ext);
+
         if (isProtocolInList(ext, ext->getProtocols(), ext->getConformances())) {
           // Break outer loop as well.
           stack.clear();