<rdar://problem/13540899> Collect using directives from all of the semantic...

<rdar://problem/13540899> Collect using directives from all of the semantic contexts not represented by scopes.

This fixes a regression I introduced in r178136, where we would not
consider the using directives from the semantic declaration contexts
that aren't represented by the lexical scopes (Scope) when performing
unqualified name lookup. This lead to horribly funny diagnostics like
"no identifier named 'foo'; did you mean 'foo'?".

llvm-svn: 179067