Parse support for tuple patterns in var and let (#3448)

Co-authored-by: Richard Smith <richard@metafoo.co.uk>

toolchain/check/handle_binding_pattern.cpp

@@ -150,6 +150,7 @@ auto HandleBindingPattern(Context& context, Parse::NodeId parse_node) -> bool {
 }
 
 auto HandleTemplate(Context& context, Parse::NodeId parse_node) -> bool {
+  // TODO: diagnose if this occurs in a `var` context.
   return context.TODO(parse_node, "HandleTemplate");
 }