Revert "Sema: err_after_alias is unreachable, remove it"

This reverts commit r226626.  err_after_alias is, in fact, reachable.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@226633 91177308-0d34-0410-b5e6-96231b3b80d8