cgen: fix interface type generation for generic struct

[felipensp]

Fix #22186

[StunxFS]

I think IB should necessarily be IB[T].

struct Foo[T] implements IA, IB[T] { 
         a int 
         b T 
 }

[felipensp]

I think IB should necessarily be IB[T].

struct Foo[T] implements IA, IB[T] { 
         a int 
         b T 
 }

What do you think @medvednikov ? This can be threated in another PR though.

[medvednikov]

I agree, it should be struct Foo[T] implements IA, IB[T]

[felipensp]

I agree, it should be struct Foo[T] implements IA, IB[T]

I've changed the implements_type to use TypeNode to save the token pos for checker error message.

[spytheman]

Excellent work.

[spytheman]

Can I merge this now @felipensp , and then #22218 or ?