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

pombredanne/go-deleteme,gnanderson/go,gnanderson/go,pombredanne/go-deleteme,apparentlymart/golang,ssrl/go,apparentlymart/golang,tv42/old-go,dsriram/golang,ssrl/go,pombredanne/go-deleteme,apparentlymart/golang,tv42/old-go,gnanderson/go,tv42/old-go,apparentlymart/golang,gnanderson/go,tv42/old-go,ssrl/go,dsriram/golang,gnanderson/go,apparentlymart/golang,ssrl/go,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,ssrl/go,tv42/old-go,dsriram/golang,ssrl/go,pombredanne/go-deleteme,gnanderson/go,apparentlymart/golang,pombredanne/go-deleteme,dsriram/golang,dsriram/golang,dsriram/golang,ssrl/go,gnanderson/go,apparentlymart/golang,dsriram/golang