Merge attributes on typedef decls.

Not completely sure this is right, but it's clearly better than what
we did before this commit (effectively dropping the attribute).

<rdar://problem/14413117>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@186373 91177308-0d34-0410-b5e6-96231b3b80d8