The overloaded `operator&` caused non-conforming behavior when
- using `operator==` to compare "addresses" of proxy reference objects,
and
- relying on the exact type of `&ref`.
No deprecation warning is added, becaue it should be portable to write
`&ref` where `ref` is a proxy reference variable, and this patch just
corrects the behavior.
`__bit_const_reference::operator&` is kept, because when one defines
`_LIBCPP_ABI_BITSET_VECTOR_BOOL_CONST_SUBSCRIPT_RETURN_BOOL` to make the
libc++ implementation strategy conforming, the `operator&` will never be
exposed to users.
2e02135cec