Define GenericParamListRequest

GenericParamListRequest formalizes the lazy generic parameter list construction pattern we were performing before.

lib/Serialization/Serialization.cpp

@@ -3081,7 +3081,6 @@ public:
                                rawAccessLevel, proto->getInherited().size(),
                                inheritedAndDependencyTypes);
 
-    const_cast<ProtocolDecl*>(proto)->createGenericParamsIfMissing();
     writeGenericParams(proto->getGenericParams());
     S.writeGenericRequirements(
       proto->getRequirementSignature(), S.DeclTypeAbbrCodes);