swift234

[Sema] implement better type circularity check

Certain type circularities weren't being checked until this point. Such as

```
struct X<T> { let a: X<X> }
struct Y<T> { let a: (Int, Y<Y>) }
enum Z<T> { case A(Optional<Z<Z>>) }
```

We introduce a more comprehensive approach to detect these in type checker.
After name lookup, exam each value field in declared structs and enums for
self-reference types that creates inifinite sizes.