go/types, types2: better error message for invalid type parameter term

The spec says "In a union, a term cannot be a type parameter,...",
but it's really the type in a term that cannot be a type parameter.
(Also, for the spec's purposes, a single term is still a union.)

This CL changes the current error message from:

        "cannot use type parameter in typeset"

to one of two messages:

        "term cannot be a type parameter"           (for term of form P)
        "type in term ~P cannot be a type parameter (for term of form ~P)

which are more specific and match the spec more closely.

Fixes #50420.

Change-Id: Id48503efc8416cabc03d5c40d8e64d5b3a7f078e
Reviewed-on: https://go-review.googlesource.com/c/go/+/396874
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

0 files changed, 0 insertions, 0 deletions