[libc++] Fix ABI break in __bit_reference.
The libc++ __bit_iterator type has weird ABI calling conventions as a
of the implementation. The const bit iterator is trivial, but the
bit iterator is not because it declares a user-defined copy constructor.
Changing this now is an ABI break, so this test ensures that each type
is trivial/non-trivial as expected.
The definition of 'non-trivial for the purposes of calls':
A type is considered non-trivial for the purposes of calls if: * it has a non-trivial copy constructor, move constructor, or destructor, or * all of its copy and move constructors are deleted.