Consistently use a Witness rather than an expression to represent a possibly-symbolic witness. (#2245)

Previously we used an expression in some places and a `Witness` values in others. The eventual goal is to make `Witness` values behave like other symbolic values such as `NominalClassType`, but the first step is to consistently treat them like values rather than expressions.

No functionality change intended.

explorer/ast/expression.cpp

@@ -258,8 +258,9 @@ void Expression::Print(llvm::raw_ostream& out) const {
       break;
     }
     case ExpressionKind::InstantiateImpl: {
-      const auto& inst_impl = cast<InstantiateImpl>(*this);
-      out << "instantiate " << *inst_impl.generic_impl();
+      // TODO: For layering reasons, we can't print out the witness and
+      // argument values from here.
+      out << "instantiate impl";
       break;
     }
     case ExpressionKind::UnimplementedExpression: {