Fix name lookup with dependent using decls.

We previously mishandled UnresolvedUsingValueDecls in
NamedDecl::declarationReplaces, which caused us to forget decls
when there are multiple dependent using decls for the same name.

Fixes PR16936.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@188737 91177308-0d34-0410-b5e6-96231b3b80d8