[Serialization] Allow unbound generic types to cross-reference typealias decls

Ideally `UnboundGenericType` should never be serialized but it is
currently allowed to make generic `typealias` declarations without
specifying generic parameters, so it should be allowed to cross
reference typealias decls in such types as well because `NameAliasType`
can't be used until generic parameters are resolved.

This is only a temporary fix and more comprehensive solution is still
pending here, most likely such declarations should not produce
`UnboundGenericType` but instead should copy generic parameters from
underlying type and produce proper `NameAliasType`.

Resolves: rdar://problem/37384120

lib/Serialization/Serialization.cpp

@@ -3943,7 +3943,9 @@ void Serializer::writeType(Type ty) {
 
     unsigned abbrCode = DeclTypeAbbrCodes[UnboundGenericTypeLayout::Code];
     UnboundGenericTypeLayout::emitRecord(Out, ScratchRecord, abbrCode,
-                                         addDeclRef(generic->getDecl()),
+                                         addDeclRef(generic->getDecl(),
+                                                    /*forceSerialization*/false,
+                                                    /*allowTypeAliasXRef*/true),
                                          addTypeRef(generic->getParent()));
     break;
   }