`A8-4-8`: Allow output parameters in assign operators

[lcartey]

Affected rules

• A8-4-8

Description

Output parameters are required for user defined assign operators, and so should not be reported.

Example

constexpr byte& operator|=(byte& lhs, const byte rhs) noexcept {
  lhs = (lhs | rhs);
  return lhs;
}

[rvermeulen]

Cannot reproduce this case. Needs further discussion.