[dev.typeparams] cmd/compile/internal/types2: better error message for invalid ... use

This partially addresses the issue below: In many (all) cases we want to
handle invalid ... use in the parser as a syntax error; but this ensures
that we get a decent error if we get here anyway.

Updates #43680.

Change-Id: I93af43a5f5741d8bc76e7a13c0db75e6edf43111
Reviewed-on: https://go-review.googlesource.com/c/go/+/283475


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