Covers a missing case in codable synthesis for enums with argument
payloads that have internal and external labels. When the name of the
var decl is used, the internal name of the parameter becomes the key
instead of the API name. In this case, this causes an invalid reference
to an enum case with the internal name as an argument to be synthesized
in the derived Decodable conformance which (hopefully) crashes
downstream.
rdar://86339848
