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

d0f/go-zh,npe9/noahevans-go9,bryanxu/go-zh,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,d0f/go-zh,Triskite/willstone-goclone,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,scirelli/scirelli-go,sanjosh/sanjos100-tipc,Triskite/willstone-goclone,mhennings/marcohennings-go,Triskite/willstone-goclone,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,scirelli/scirelli-go,scirelli/scirelli-go,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,glycerine/jeaten-go-arrayof-structof,npe9/noahevans-go9,npe9/noahevans-go9,webfd/go-zh,bryanxu/go-zh,mhennings/marcohennings-go,mhennings/marcohennings-go,rflanagan/reginaldflanagan-project1,scirelli/scirelli-go,mhennings/marcohennings-go,bryanxu/go-zh,scirelli/scirelli-go,rdp/rogerpack2005-golang,d0f/go-zh,webfd/go-zh,rflanagan/reginaldflanagan-project1,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,bryanxu/go-zh,scirelli/scirelli-go,rflanagan/reginaldflanagan-project1,glycerine/jeaten-go-arrayof-structof,scirelli/scirelli-go,sanjosh/sanjos100-tipc,rflanagan/reginaldflanagan-project1,mhennings/marcohennings-go,webfd/go-zh,npe9/noahevans-go9,d0f/go-zh,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,npe9/noahevans-go9,npe9/noahevans-go9,rdp/rogerpack2005-golang,bryanxu/go-zh,webfd/go-zh,npe9/noahevans-go9,rflanagan/reginaldflanagan-project1,rflanagan/reginaldflanagan-project1,webfd/go-zh,d0f/go-zh,mhennings/marcohennings-go,d0f/go-zh,webfd/go-zh,bryanxu/go-zh,scirelli/scirelli-go,sanjosh/sanjos100-tipc,mhennings/marcohennings-go,rdp/rogerpack2005-golang,rflanagan/reginaldflanagan-project1,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,Triskite/willstone-goclone,sanjosh/sanjos100-tipc,npe9/noahevans-go9,webfd/go-zh,bryanxu/go-zh,sanjosh/sanjos100-tipc,Triskite/willstone-goclone,rflanagan/reginaldflanagan-project1,bryanxu/go-zh,webfd/go-zh,Triskite/willstone-goclone,Triskite/willstone-goclone,Triskite/willstone-goclone