They might have different visibility, and thus discarding all but one of them
can result in rejecting valid code. Also fix name lookup to cope with multiple
using-directives being found that denote the same namespace, where some are not
visible -- don't cache an "already visited" state for a using-directive that we
didn't visit because it was hidden.
git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@316965 91177308-0d34-0410-b5e6-96231b3b80d8
Richard Smith