x/debug: support changes to the internal representation of interfaces' types.

There are now typedefs in different places in the DWARF type data, and
the type name is now stored as a string instead of a *string.  This
supports both.

Change-Id: I6f963d34a0157f76a8b3d92ef998712066806c3f
Reviewed-on: https://go-review.googlesource.com/20720
Reviewed-by: Dave Day <djd@golang.org>