Add support for ->? return forms (#6849)

This includes checking and lowering for concrete form literals. Support
for symbolic forms is future work.

---------

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

toolchain/check/handle_binding_pattern.cpp

@@ -340,6 +340,9 @@ static auto HandleAnyBindingPattern(Context& context, Parse::NodeId node_id,
       context.node_stack().Push(node_id, binding_pattern_id);
       break;
     }
+
+    case FullPatternStack::Kind::NotInEitherParamList:
+      CARBON_FATAL("Unreachable");
   }
   return true;
 }