[libc++] Fix -Wdeprecated-copy warnings in __bit_reference

[echristo]

Summary

Add a couple of default copy constructors to fix the warning.

[dblaikie]

Sounds good to me.

(my thought process, for posterity (beyond the fact that this code change makes explicit what was already happening/it doesn't change behavior): copy constructing a reference binds the new copy to the same entity as the original. This is different from copy assigning, which doesn't bind/rebind, but instead changes the value of the object referenced by the LHS reference to match the value of the object referenced by the RHS reference - so it seems good/proper that these types would have a non-trivial copy assignment, while wanting to opt back into the default copy construction)

[echristo]

clang-format changed lines.

[merge_guards_bot]

clang-format: pass.

Build artifacts: console-log.txt, diff.json

[merge_guards_bot]

Unit tests: pass. 60748 tests passed, 0 failed and 726 were skipped.

clang-format: pass.

Build artifacts: console-log.txt, CMakeCache.txt, test-results.xml, diff.json

[echristo]

Reopening this.

Caused some failures due to public/private and the function was actually called in a number of places where it would need to be public. I'm not sure if we wanted to make this public... or do something else so reopening :)

[mclow.lists]

It appears that we have multiple things in review here.
See also D71096.
Also, is this in response to https://bugs.llvm.org/show_bug.cgi?id=44145 ? (which was closed as 'works for me')

[echristo]

Abandoning this one in favor of maskray's revision.

[echristo]

In D71395#1781896, @mclow.lists wrote:

It appears that we have multiple things in review here.
See also D71096.
Also, is this in response to https://bugs.llvm.org/show_bug.cgi?id=44145 ? (which was closed as 'works for me')

It is, but I'm still seeing it on every build so getting D71096 approved would be great. Thanks :)

libcxx/include/__bit_reference

public:
     {
         if (__x)
             *__seg_ |= __mask_;
         else
             *__seg_ &= ~__mask_;
         return *this;
     }
 
+    __bit_reference(const __bit_reference&) = default;
+
     _LIBCPP_INLINE_VISIBILITY
     __bit_reference& operator=(const __bit_reference& __x) _NOEXCEPT
         {return operator=(static_cast<bool>(__x));}
 
     _LIBCPP_INLINE_VISIBILITY void flip() _NOEXCEPT {*__seg_ ^= __mask_;}
     _LIBCPP_INLINE_VISIBILITY __bit_iterator<_Cp, false> operator&() const _NOEXCEPT
         {return __bit_iterator<_Cp, false>(__seg_, static_cast<unsigned>(__libcpp_ctz(__mask_)));}
 private:
_LIBCPP_INLINE_VISIBILITY __bit_iterator<_Cp, true> operator&() const _NOEXCEPT
         {return __bit_iterator<_Cp, true>(__seg_, static_cast<unsigned>(__libcpp_ctz(__mask_)));}
 private:
     _LIBCPP_INLINE_VISIBILITY
     _LIBCPP_CONSTEXPR
     __bit_const_reference(__storage_pointer __s, __storage_type __m) _NOEXCEPT
         : __seg_(__s), __mask_(__m) {}
 
     __bit_const_reference& operator=(const __bit_const_reference& __x);
+    __bit_const_reference(const __bit_const_reference&) = default;
 };
 
 // find
 
 template <class _Cp, bool _IsConst>
 __bit_iterator<_Cp, _IsConst>
 __find_bool_true(__bit_iterator<_Cp, _IsConst> __first, typename _Cp::size_type __n)
 {