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

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

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@245335 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed