[BitmaskEnum] Add support for shift operators. (#118007)

For enums that describe a bitmask where successive bits within that mask
describe some enum value (as described in the same enum), it is useful
to support operator<< and operator>> as well.

For example:
```
  enum class E : unsigned {
    // 2 bits per option
    OptionA = 0,
    OptionB = 1,
    OptionC = 2,
    OptionD = 3,
    OptionMask = 3,

    // Given 3 values in the bitmask X, Y and Z, each is 2 bits in size
    // and represents a choice of OptionA..OptionD.
    ShiftX = 0,
    ShiftY = 2,
    ShiftZ = 4,
  };

  // The mask can be encoded with:
  E mask;
  mask |= getOptionFor(X) << E::ShiftX;
  mask |= getOptionFor(Y) << E::ShiftY;
  mask |= getOptionFor(Z) << E::ShiftZ;

  // And to extract a value:
  E OptionForX =  (mask >> E::ShiftX) & E::OptionMask;
  E OptionForY =  (mask >> E::ShiftY) & E::OptionMask;
  E OptionForZ =  (mask >> E::ShiftZ) & E::OptionMask;
```