[ItaniumDemangle] Add Named flag to "pm" operator (#136862)

Compilers can generate mangled names such as `_ZN1CpmEi` which we
currently fail to demangle. The OperatorInfo table only marked the `pt`
operator as being "named", which prevented the others from demangling
properly. Removing this logic for the other kinds of member operators
isn't causing any tests to fail.

GitOrigin-RevId: 028f70d2524be56bb5d191ec3d7022b40bc4859b