__bit_reference: fix -Wdeprecated-copy warnings

Since C++11, [depr.impldec]:

The implicit definition of a copy constructor as defaulted is deprecated
if the class has a user-declared copy assignment operator or a
user-declared destructor.

At clang HEAD, -Wdeprecated-copy (included by -Wextra) will warn on such instances.

Reviewed By: EricWF

Differential Revision: https://reviews.llvm.org/D71096