Serialization: When deserializing a typealias, build its generic signature

We were forgetting to do this, triggering crashes when using a
generic typealias from another module.

Fixes <https://bugs.swift.org/browse/SR-1889>.

lib/Serialization/Serialization.cpp

@@ -2191,6 +2191,7 @@ void Serializer::writeDecl(const Decl *D) {
                                 typeAlias->isImplicit(),
                                 rawAccessLevel);
     writeGenericParams(typeAlias->getGenericParams(), DeclTypeAbbrCodes);
+    writeRequirements(typeAlias->getGenericRequirements());
     break;
   }