cmd/compile/internal/syntax: better error message when type parameters are not permitted

Fixes #48382.

Change-Id: I215896a4429839c41c9136b6922b1b748ed47734
Reviewed-on: https://go-review.googlesource.com/c/go/+/361259


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>