[dev.regabi] go/types: type alias decl requires go1.9

This is a port of CL 289570 to go/types. It has some notable differences
with that CL:
 + A new _BadDecl error code is added, to indicate declarations with bad
   syntax.
 + declInfo is updated hold not an 'alias' bool, but an aliasPos
   token.Pos to identify the location of the type aliasing '=' token.
   This allows for error messages to be accurately placed on the '='

For #31793

Change-Id: Ib15969f9cd5be30228b7a4c6406f978d6fc58018
Reviewed-on: https://go-review.googlesource.com/c/go/+/291318


Trust: Robert Findley <rfindley@google.com>
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>