[libc++] Support immovable return types in std::function.

LWG reflector consensus is that this was a bug in libc++.
(In particular, MSVC also will fix it in their STL, soon.)
Bug originally discovered by Logan Smith.

Also fix `std::function<const void()>`, which should work
the same way as `std::function<void()>` in terms of allowing
"conversions" from non-void types.

Differential Revision: https://reviews.llvm.org/D94452