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

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

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D71395

Author: echristo

libcxx/include/__bit_reference

@@ -62,6 +62,8 @@ public:
         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));}
@@ -148,6 +150,7 @@ private:
         : __seg_(__s), __mask_(__m) {}
 
     __bit_const_reference& operator=(const __bit_const_reference& __x);
+    __bit_const_reference(const __bit_const_reference&) = default;
 };
 
 // find