swift234

Sema: Allow dynamic casts from generics to concrete types.

Open us 'a as! T' to allow dynamic casts from archetypes to archetypes, archetypes to concrete types, existentials to archetypes, and existentials to concrete types. When the type-checker finds these cases, generate new Unchecked*To*Expr node types for each case.

We don't yet check whether the target type actually makes sense with the constraints of the archetype or existential, nor do we implement the SILGen/IRGen backends for these operations. We also don't extend 'x is T' to query the new operation kinds. There's a better factoring that would allow 'as!' and 'is' to share more code. For now, I want to make sure 'x as! T' continues to work for ObjC APIs when we flip the switch to import protocol types.

Swift SVN r5611