Fix bug in using shadow decl checking: a using shadow decl should not conflict

Fix bug in using shadow decl checking: a using shadow decl should not conflict
with a prior UsingDecl -- those should not even really be found by the lookup
here, except that we use the same lookup results for two different checks, and
the other check needs them.

This happens to work in *almost all* cases, because either the lookup results
list the UsingDecl first (and the NonTag result gets replaced by something
else) or because the problematic declaration is a function (which causes us to
use different logic to detect conflicts). This can also be triggered from a
state only reachable through modules (where the name lookup results can contain
multiple UsingDecls in the same scope).

llvm-svn: 262105