Make `#[MustUseResult]` work with inheritance

[ZebulanStanphill]

I have repeatedly run into situations where I want to declare an interface method as requiring the result to be used, but currently, the #[MustUseResult] attribute is not inherited, so I have to place the attribute on every implementation of the method.

Use-cases include nearly every method where the attribute would normally be used, where such method is originally defined on an interface, abstract class, or parent class.

Perhaps, for the rare case where an overriding method differs from its parent in that its return value is probably ignorable, it might make sense to add an optional parameter to the attribute to flip the behavior e.g. #[MustUseResult(false)]. I would not consider this essential, however.

See also #29, which is basically this issue but for the #[Friend] attribute instead.

[DaveLiddament]

@ZebulanStanphill I've created a PR on the PHPStan extension.

Are you able to check this is correct for your use case? DaveLiddament/phpstan-php-language-extensions#38

You can see the test cases:

• https://github.com/DaveLiddament/phpstan-php-language-extensions/blob/feature/respect-must-use-result-on-parent-methods/tests/Rules/data/mustUseResult/mustUseResultOnInterface.php
• https://github.com/DaveLiddament/phpstan-php-language-extensions/blob/feature/respect-must-use-result-on-parent-methods/tests/Rules/data/mustUseResult/mustUseResultOnExtendedClass.php