Build value representations for all kinds of witness (#2250)

Instead of forming a `SymbolicWitness` that contains an `Expression` when we can't directly resolve a witness to an `impl`, form different kinds of `Witness` values for the various situations:

- A `BindingWitness` witnesses that a type implements a constraint by reference to an `ImplBinding`.
- A `ConstraintWitness` witnesses that a type implements a constraint by reference to witnesses for each of the impl constraints within the constraint.
- A `ConstraintImplWitness` witnesses that a type implements a constraint by reference to a larger constraint which contains that constraint as an impl constraint.

Remove `SymbolicWitness` values and `InstantiateImpl` expressions, which are now unused. In order to remove the final usage of `SymbolicWitness`, I fixed a TODO to give `Self` the proper type within an interface declaration. I don't think this is an observable change.

No functional change intended.

explorer/ast/expression.cpp

@@ -257,12 +257,6 @@ void Expression::Print(llvm::raw_ostream& out) const {
       }
       break;
     }
-    case ExpressionKind::InstantiateImpl: {
-      // TODO: For layering reasons, we can't print out the witness and
-      // argument values from here.
-      out << "instantiate impl";
-      break;
-    }
     case ExpressionKind::UnimplementedExpression: {
       const auto& unimplemented = cast<UnimplementedExpression>(*this);
       out << "UnimplementedExpression<" << unimplemented.label() << ">(";
@@ -347,7 +341,6 @@ void Expression::PrintID(llvm::raw_ostream& out) const {
     case ExpressionKind::UnimplementedExpression:
     case ExpressionKind::FunctionTypeLiteral:
     case ExpressionKind::ArrayTypeLiteral:
-    case ExpressionKind::InstantiateImpl:
       out << "...";
       break;
   }