Used by DWARF writer changes in a followup CL.

Change-Id: I6ec40dcfeaba909d9b8f6cf2603bc5b85c1fa873
Reviewed-on: https://go-review.googlesource.com/20073Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>