go/ast: predeclared objects have the Universe/Unsafe scope as Decl

Makes it possible to easily detect if an Object was predeclared
(as opposed to unresolved).

R=rsc
CC=golang-dev
https://golang.org/cl/5530072

src/pkg/exp/types/universe.go

@@ -20,6 +20,7 @@ func define(kind ast.ObjKind, name string) *ast.Object {
 	if scope.Insert(obj) != nil {
 		panic("types internal error: double declaration")
 	}
+	obj.Decl = scope
 	return obj
 }
 

src/pkg/go/ast/scope.go

@@ -80,7 +80,7 @@ func (s *Scope) String() string {
 type Object struct {
 	Kind ObjKind
 	Name string      // declared name
-	Decl interface{} // corresponding Field, XxxSpec, FuncDecl, LabeledStmt, or AssignStmt; or nil
+	Decl interface{} // corresponding Field, XxxSpec, FuncDecl, LabeledStmt, AssignStmt, Scope; or nil
 	Data interface{} // object-specific data; or nil
 	Type interface{} // place holder for type information; may be nil
 }
@@ -131,6 +131,8 @@ func (obj *Object) Pos() token.Pos {
 				return ident.Pos()
 			}
 		}
+	case *Scope:
+		// predeclared object - nothing to do for now
 	}
 	return token.NoPos
 }