<vector>: _Apply_annotation() should be able to _Unfancy()

[StephanTLavavej]

After we merge #2464, @CaseyCarter observed in #2464 (comment) that it should be possible to change _Apply_annotation()'s uses of _Unfancy_maybe_null() to _Unfancy():

STL/stl/inc/vector

Lines 557 to 560 in 0dc93fc

	const auto _First = reinterpret_cast<const char*>(_Unfancy_maybe_null(_First_));
	const auto _End = reinterpret_cast<const char*>(_Unfancy_maybe_null(_End_));
	const auto _Old_last = reinterpret_cast<const char*>(_Unfancy_maybe_null(_Old_last_));
	const auto _New_last = reinterpret_cast<const char*>(_Unfancy_maybe_null(_New_last_));

I'm filing a followup issue as #2464 is a high-priority fix that we don't want to delay for a cleanup, however worthy it may be.