Adding virtual_overrider_self_life_support. (#2902)

* Initial version of virtual_overrider_self_life_support (enables safely passing unique_ptr to C++).

* Clang 3.6, 3.7 compatibility.

* Adding missing default constructor.

* Restoring test for exception for the case that virtual_overrider_self_life_support is not used.

* Fixing oversight: Adding missing holder().ensure_was_not_disowned().

* Adding unit tests for new `struct smart_holder` member functions.

* Moving virtual_overrider_self_life_support to separate include file, with iwyu cleanup.