<vector>: Cast to the correct base type in vector<bool, Alloc>::_Orphan_range_unlocked

[frederick-vs-ja]

I've confirmed that @StephanTLavavej's analysis for the related test failure was correct.

Possible STL bug, _Vb_reference derives from _Vb_iter_base and is adopted by the container, but _Orphan_range_unlocked assumes every child is a const_iterator.

This PR exposes _Mybase for the convenience of casting, and removes some unnecessary _Difference_type member type alias as driven-by changes. (It seems better to me to use difference_type whenever it's available.)

Unblocks one libcxx test:

• std/containers/sequences/vector.bool/emplace_back.pass.cpp

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[StephanTLavavej]

Thanks for confirming my initial investigation and figuring out how to fix this libcxx failure! 📉 😻 🛠️