When tag lookup finds something ambiguous, and we're defining a new

tag, filter out those ambiguous names that we found if they aren't
within the declaration context where this newly-defined tag will be
visible.

This is basically a hack, because we really need to fix the lookup of
tag declarations in this case to not find things it
shouldn't. However, it's better than what we had before, and it fixes
<rdar://problem/9168556>. 

llvm-svn: 130810