S873: Allow bit field operations for enums

Currently, this rule is very verbose, and most findings are related to bitfield operations performed on enums. |, &, |=, &=, (maybe more surprisingly) <<.

Maybe we should allow them unconditionally, or at least allow them as soon as some enumerators had a non default initializer?

Or maybe we should remain strict, but then suggests the user that he should define overloaded operators to perform these operations? => In some cases, you want the result of this combination to be an int, so you don't need a user-defined operator