clang can emit some dwarf.VoidType which are wrapped by multiple
dwarf.TypedefType. We need to unwrap those before further processing.

Fixes #20129

Change-Id: I671ce6aef2dc7b55f1a02aec5f9789ac1b369643
Reviewed-on: https://go-review.googlesource.com/44772Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>