Description:
------------
Split off from bug #69419.

Generators use the ampersand normally indicating by-reference returns to signify by-reference yields. Currently the (covariant) LSP checks for by-ref returns does not take this into account.

When a generator extends another generator, this behavior results in the correct semantics:

class A {
    public function &gen() { yield; }
}
class B extends A {
    public function gen() { yield; }
}

Here B::gen() is indeed not compatible with A::gen(). However when mixing generators and normal iterators this is not the case:

class A {
    public function &gen() { yield; }
}
class B extends A {
    public function gen() { return ArrayIterator([]); }
}

This code should be allowed (without the strict standards notice), because B::gen() can also be iterated by-reference.

The LSP check should be adjusted for the case where the prototype is a generator.