cue/token: record File pointer in Pos

This is the first step to eliminate FileSet.

Change-Id: Ibb8a17e429d9b2dc2fda0a739520ebef40c67761
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/2121
Reviewed-by: Marcel van Lohuizen <mpvl@google.com>
diff --git a/internal/protobuf/util.go b/internal/protobuf/util.go
index e001fa8..980a7d3 100644
--- a/internal/protobuf/util.go
+++ b/internal/protobuf/util.go
@@ -38,8 +38,8 @@
 }
 
 var (
-	newline    = token.Pos(token.Newline)
-	newSection = token.Pos(token.NewSection)
+	newline    = token.Newline.Pos()
+	newSection = token.NewSection.Pos()
 )
 
 func addComments(f ast.Node, i int, doc, inline *proto.Comment) bool {