go/parser: always introduce an ast.Object when declaring an identifier
go/parser: always introduce an ast.Object when declaring an identifier

When traversing parameter lists (e.g. for type checking), we want the
invariant that all identifers have associated objects (even _ idents),
so that we can associate a type with each object.

R=rsc
CC=golang-dev
http://codereview.appspot.com/4490042

wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,wesolows/golang,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old