[libcxx] Fix PR23589: std::function doesn't recognize null pointer to varargs function.

Summary:
This patch fixes __not_null's detection of nullptr by breaking it down into 4 cases.

1. `__not_null(Tp const&)`: Default case. Tp is not null.
2. `__not_null(Tp* __ptr);` Case for pointers to functions.
3. `__not_null(_Ret _Class::* __ptr);` Case for pointers to members.
4. `__not_null(function<Tp> const&);`: Cases for other std::functions.

Reviewers: mclow.lists

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D11111

llvm-svn: 245335