go.tools/go/types: introduce Func.Scope()

The goal is to allow clients to distinguish function-local objects
from package-local objects and identify the specific function by
examining its scope. For example, a compiler may need to mangle
object names according to the outer function name.

R=gri
CC=golang-codereviews, golang-dev
https://golang.org/cl/52280043

go/types/objects.go

@@ -192,6 +192,10 @@ func (obj *Func) FullName() string {
 	return buf.String()
 }
 
+func (obj *Func) Scope() *Scope {
+	return obj.typ.(*Signature).scope
+}
+
 // A Label represents a declared label.
 type Label struct {
 	object