LWG-3778 `vector<bool>` missing exception specifications

[StephanTLavavej]

LWG-3778 vector<bool> missing exception specifications

We have some strengthening, but we need to align this with the Standard. (It's okay if we end up with properly-commented strengthening beyond what this LWG issue resolution provides.)

[frederick-vs-ja]

Allocator rebinding wasn't discussed in WG21-N4258, and implementations seemly assume that rebound allocators have the same properties of propagation as user-provided allocators. It seems that we can't reasonably implement the standard requirements if _Choose_pocma_v<_Alvbase> is _No_propagate_allocators while _Choose_pocma_v<_Alloc> has a different value.

I think it may be necessary to resolve LWG-3267 more completely by requiring that

• is_always_equal::value,
• propagate_on_container_copy_assignment::value,
• propagate_on_container_move_assignment::value, and
• propagate_on_container_swap::value

are all equal for allocator_traits<X> and allocator_traits<Y>.