If a function type has no type parameters, note when it
is visited and do not recur. (It must be visited
at least once because of closures and their associated
types occurring in a generic context).
Fixes#51832.
Change-Id: Iee20612ffd0a03b838b9e59615f4a0206fc8940b
Reviewed-on: https://go-review.googlesource.com/c/go/+/406714
Reviewed-by: Keith Randall <khr@google.com>
