Add an associated entity instruction and corresponding type for interface elements. (#3730)

When declaring an associated entity in an interface -- just associated
functions for now -- create an associated entity value and corresponding
type to represent a "slot in a witness table". Also track the list of
associated entities on the interface so that we will eventually be able
to check impls against them.

Associated entities are represented as the integer index of their slot
in a witness table.

---------

Co-authored-by: Jon Ross-Perkins <jperkins@google.com>

toolchain/check/handle_interface.cpp

@@ -130,7 +130,9 @@ auto HandleInterfaceDefinitionStart(
 
   context.inst_block_stack().Push();
   context.node_stack().Push(parse_node, interface_id);
-  // TODO: Perhaps use the args_type_info_stack for a witness table.
+
+  // We use the arg stack to build the witness table type.
+  context.args_type_info_stack().Push();
 
   // TODO: Handle the case where there's control flow in the interface body. For
   // example:
@@ -153,10 +155,13 @@ auto HandleInterfaceDefinition(Context& context,
   context.inst_block_stack().Pop();
   context.scope_stack().Pop();
   context.decl_name_stack().PopScope();
+  auto associated_entities_id = context.args_type_info_stack().Pop();
 
   // The interface type is now fully defined.
   auto& interface_info = context.interfaces().Get(interface_id);
-  interface_info.defined = true;
+  if (!interface_info.associated_entities_id.is_valid()) {
+    interface_info.associated_entities_id = associated_entities_id;
+  }
   return true;
 }