Make sure code that is in the middle of figuring out the correct architecture

on attach uses the architecture it has figured out, rather than the Target's
architecture, which may not have been updated to the correct value yet.

<rdar://problem/24632895>

Author:    Jim Ingham <jingham@apple.com>
Date:      Thu Feb 18 23:58:45 2016 +0000

This is a cherry-pick of r261279

llvm-svn: 270116