Add support for derived class special members hiding functions brought in from

Add support for derived class special members hiding functions brought in from
a base class via a using-declaration. If a class has a using-declaration
declaring either a constructor or an assignment operator, eagerly declare its
special members in case they need to displace a shadow declaration from a
using-declaration.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@269398 91177308-0d34-0410-b5e6-96231b3b80d8