Associated constants can be used in member function signatures (#5089)

This required allowing incomplete facet types where previously
completeness was required. Once we support named constraints, we will
need a way to consistently go from an interface to a facet type witness
index without requiring the interface to be complete in these cases.

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Co-authored-by: Josh L <josh11b@users.noreply.github.com>

toolchain/check/handle_interface.cpp

@@ -154,6 +154,9 @@ auto HandleParseNode(Context& context,
   interface_info.definition_id = interface_decl_id;
   interface_info.scope_id = context.name_scopes().Add(
       interface_decl_id, SemIR::NameId::None, interface_info.parent_scope_id);
+  context.name_scopes()
+      .Get(interface_info.scope_id)
+      .set_is_interface_definition();
 
   auto self_specific_id =
       context.generics().GetSelfSpecific(interface_info.generic_id);