Rename ToString to mention Operator (#2539)

Naming this ToString within the Carbon namespace encourages incidental overloading of the function, and that doesn't seem to be intentional; this feels questionable [under overloading style](https://google.github.io/styleguide/cppguide.html#Function_Overloading). OperatorToString seems helpful in that it makes it easy to see at a glance where it's being used.

explorer/interpreter/interpreter.cpp

@@ -253,7 +253,7 @@ auto Interpreter::EvalPrim(Operator op, Nonnull<const Value*> /*static_type*/,
     case Operator::BitShiftLeft:
     case Operator::BitShiftRight:
     case Operator::Complement:
-      CARBON_FATAL() << "operator " << ToString(op)
+      CARBON_FATAL() << "operator " << OperatorToString(op)
                      << " should always be rewritten";
   }
 }