Work around broken Apple clang implementation

This line of code does not cause an error on clang-3.4 or clang-3.5
on Linux. Apple's clang is somewhere between the two, no way to know
where.

For an unknown reason, specifying default move operations in 'Data'
causes the compiler to think that Boxed_Value is an incomplete type.

This is highly illogical since Data is only used via a shared_ptr, so
the size / type of Boxed_Value should be fully known (and is known on
every other compiler/platform combination).