go/ast, parser: remember short variable decls. w/ correspoding ident objects

go/ast, parser: remember short variable decls. w/ correspoding ident objects

The ast.Object's Decl field pointed back to the corresponding declaration for
all but short variable declarations. Now remember corresponding assignment
statement in the Decl field.

Also: simplified some code for parsing select statements.

R=golang-dev, r, bradfitz
CC=golang-dev
https://golang.org/cl/5492072