Sema: A couple of recursion-breaking hacks

lib/AST/Module.cpp

@@ -599,6 +599,10 @@ ModuleDecl::lookupConformance(Type type, ProtocolDecl *protocol,
   // existential's list of conformances and the existential conforms to
   // itself.
   if (type->isExistentialType()) {
+    // FIXME: Recursion break.
+    if (!protocol->hasValidSignature())
+      return None;
+
     // If the existential type cannot be represented or the protocol does not
     // conform to itself, there's no point in looking further.
     if (!protocol->existentialConformsToSelf() ||