go/types: report error for invalid (but empty) expr switch
This is a port of CL 278132 from the dev.typeparams branch. A notable
addition is a new error code, since no existing codes made sense and we
have an analogous code for type switches.
Fixes #43110
Change-Id: I22b3f9d8777063223f82785504e8b7d299bc5216
Reviewed-on: https://go-review.googlesource.com/c/go/+/278813
Run-TryBot: Robert Findley <rfindley@google.com> Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Robert Findley <rfindley@google.com>