spec: the type of a constant cannot be a type parameter

Add corresponding rules and a couple of examples.

Fixes #50202.

Change-Id: I4287b5e2d0fd29a0c871795e07f1bb529c9c6004
Reviewed-on: https://go-review.googlesource.com/c/go/+/384240


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>