Mark ACTION_Pn()-generated functions as must-use-result.

This catches when a client creates an action and discards it, thinking that the action has actually been applied to something.

This will help people who make the mistake of defining, for example, both `void Use(Foo*)` and `ACTION(Use) { Use(arg); }` for later application to a Foo.  With such an overload, a client may then write `Use();`, forgetting the param and being confused why nothing happens.

This also catches when a client defines their own action in terms of an ACTION()-generated one, invokes the ACTION's builder, and then fails to invoke the resulting action, thinking it's operating on the outer action's parameters.

PiperOrigin-RevId: 312108101