tools: switch to standard go/types at tip

A few files have been forked and tagged "go1.5,!go1.6" to work around
minor API changes between the two types packages:
- constant.Value.String() in oracle/describe.go and its tests;
- constant.ToInt must now be called before constant.Int64Val.
- types.Config{Importer: importer.Default()} in a number of places
- go/types/typeutil/import_test.go uses lowercase names to avoid 'import "C"'.

Files in go/types/typesutil, missing from my previous CL, have been
tagged !go1.5; these files will be deleted in February.

All affected packages were tested using 1.4.1, 1.5, and ~1.6 (tip).

Change-Id: Iec7fd370e1434508149b378438fb37f65b8d2ba8
Reviewed-on: https://go-review.googlesource.com/18207
Reviewed-by: Robert Griesemer <gri@golang.org>
diff --git a/go/pointer/hvn.go b/go/pointer/hvn.go
index c144559..e550bc9 100644
--- a/go/pointer/hvn.go
+++ b/go/pointer/hvn.go
@@ -165,11 +165,11 @@
 
 import (
 	"fmt"
+	"go/types"
 	"io"
 	"reflect"
 
 	"golang.org/x/tools/container/intsets"
-	"golang.org/x/tools/go/types"
 )
 
 // A peLabel is a pointer-equivalence label: two nodes with the same