Scaffolding for checking impls. (#3672)

Consume the components of the `impl` declaration, and set up scopes for
the child elements. We don't yet build a representation for the impl
itself.

Also, add an interface type value. This is necessary so that we have a
value for the expression on the right-hand side of `as` in an `impl`.

toolchain/check/context.cpp

@@ -1015,6 +1015,10 @@ class TypeCompleter {
                 .object_repr_id,
             SemIR::ValueRepr::ObjectAggregate);
 
+      case SemIR::InterfaceType::Kind:
+        // TODO: Should we model the value representation as a witness?
+        return MakeEmptyValueRepr();
+
       case SemIR::Builtin::Kind:
         CARBON_FATAL() << "Builtins should be named as ImportRefUsed";