cmd/compile: disable type list syntax for the compiler

cmd/compile: disable type list syntax for the compiler

Add (temporary) syntax.AllowTypeLists mode to control the
acceptance of type lists; the compiler doesn't set it,
but existing syntax and types2 tests do so that the code
remains exercised while it exists.

Adjust various tests to use the type set notation.

Change-Id: I798e607912552db6bfe38a7cd4324b74c6bf4d95
Reviewed-on: https://go-review.googlesource.com/c/go/+/347249
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>