Fix #16287 by checking for more unsaturated synonym arguments

Trac #16287 (closed) shows that we were checking for unsaturated type synonym arguments (in :kind) when the argument was to a type synonym, but not when the argument was to some other form of type constructor, such as a data type. The solution is to use the machinery that rejects unsaturated type synonym arguments (previously confined to check_syn_tc_app) to check_arg_type, which checks these other forms of arguments. While I was in town, I cleaned up check_syn_tc_app a bit to only invoke check_arg_type so as to minimize the number of different code paths that that function could go down.