go/parser, go/types, syntax, types2: report invalid uses of ... by parsers

go/parser, go/types, syntax, types2: report invalid uses of ... by parsers

Check correct use of ...'s in parameter lists in parsers.
This allows the type checkers to assume correct ASTs with
respect to ... use.

Adjust some error messages: if a ... is used in a result
parameter list, the error is now more accurate.

Eliminate a now unused error code.

Change-Id: I66058e114e84805e24c59e570604b607ef5ff1fe
Reviewed-on: https://go-review.googlesource.com/c/go/+/631135
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Bypass: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>