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>