Sema: Make type resolution for EnumElementPattern less eager

[AnthonyLatsis]

Delay contextual type resolution for EnumElementPattern until coercePatternToType is called to further reduce the scope of UnboundGenericType, and use name lookup instead to resolve the pattern.

The definitive fix would be to move CaseStmt checking into the constraint system (AFAIK CaseStmt is the only remaining place where EnumElementPattern checking still relies on coercePatternToType).

[slavapestov]

@swift-ci Please test source compatibility

[slavapestov]

@swift-ci Please smoke test

[hamishknight]

What about cases where the contextual type is needed to resolve the case? I believe this will no longer compile with this change:

enum E {
  case e
}

struct S<T> {}
extension S where T == E {
  func foo(_ x: Any) {
    switch x {
    case T.e:
      break
    default:
      break
    }
  }
}

As you say, it would be nice if we could move this into the constraint system such that we resolve patterns while generating constraints, which would allow us to keep the generic opening contained and allow the above example to remain valid.

[AnthonyLatsis]

Argh.. well noticed. @CodaFi Should we revert this? Not sure if I can figure out the proper constraint-system-oriented fix in reasonable time, considering the regression.

[hamishknight]

Yeah IMO we should revert this if a fix for the above case isn't going to be landed soon.