go/types: remove a TODO, enable a test

1) Scopes do have a comment field for documentation (debugging output).
   No need to do anything extra.

2) The testcase in expr3.src has ok error messages. Enabled.

Change-Id: Ic1a03bfec0a6a70d876aa6cfb936973abe58fe6c
Reviewed-on: https://go-review.googlesource.com/c/139902Reviewed-by: Alan Donovan <adonovan@google.com>

src/go/types/scope.go

@@ -15,9 +15,6 @@ import (
 	"strings"
 )
 
-// TODO(gri) Provide scopes with a name or other mechanism so that
-//           objects can use that information for better printing.
-
 // A Scope maintains a set of objects and links to its containing
 // (parent) and contained (children) scopes. Objects may be inserted
 // and looked up by name. The zero value for Scope is a ready-to-use

src/go/types/testdata/expr3.src

@@ -497,7 +497,7 @@ func _calls() {
 	f1(x ... /* ERROR "cannot use ..." */ )
 	f1(g0 /* ERROR "used as value" */ ())
 	f1(g1())
-	// f1(g2()) // TODO(gri) missing position in error message
+	f1(g2 /* ERROR "cannot use g2" */ /* ERROR "too many arguments" */ ())
 
 	f2() /* ERROR "too few arguments" */
 	f2(3.14) /* ERROR "too few arguments" */