Return static instance on abstract class setters

[veewee]

Q	A
Documentation	no
Bugfix	no/yes
BC Break	no
New Feature	no
RFC	no
QA	no

Description

This PR improves static analysis when using the setters that are located in the abstract generators:
Instead of returning with type Abstract* on setters, the static instance will be returned.
This makes static analyzers understand a fluent approach better.

For example:

(new MethodGenerator('getX'))
    ->setStatic(true)
    // > This would previously result in the static analyzer to believe the type is `AbstractMemberGenerator`
    ->setBody('return '.$code)
    ->setReturnType(ClassMapCollection::class)

Resulting in issues like:

>   54     Parameter #1 $method of method
         Laminas\Code\Generator\ClassGenerator::addMethodFromGenerator()
         expects Laminas\Code\Generator\MethodGenerator,
         Laminas\Code\Generator\AbstractMemberGenerator given.

[Ocramius]

Thanks @veewee