[libcxx] Support allocators with explicit c-tors in vector<bool>

std::vector<bool> rebinds the supplied allocator to construct objects
of type '__storage_type' rather than 'bool'. Allocators are allowed to
use explicit conversion constructors, so care must be taken when
performing conversions.

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D112150