Allowing self-conformance for protocols

[xwu (Xiaodi Wu)]

Lifting the "Self or associated type" constraint on existentials

When the first seed of Swift came out many years ago, there were technical reasons for the "self or associated type" constraint on protocol existentials: At that time, protocol witness tables did not carry associated type information, so it was impossible to re-open the dynamic type in order to dispatch methods on an existential when a protocol had associated types. This was fixed a while ago in order to allow for recursive protocol constraints, but we held on to the restriction on existentials …