C++ using decls that refer to member operators in a base class are not callable

[egorzhdan]

MSVC's implementation of std::vector::iterator uses using Base::operator-;, which currently doesn't result in func -(_:std.vector.iterator,_:std.vector.iterator) being created. This operator is required for the automatic conformance to UnsafeCxxRandomAccessIterator. The lack of it prevents the conformance of std.vector to CxxRandomAccessCollection from being synthesized on Windows in #69433.