Small fix to the ast collector visitor

Small fix to the ast collector visitor

The parameter type was used instead of the default value.

gcc/rust/ChangeLog:

	* ast/rust-ast-collector.cc (TokenCollector::visit): Check for presence
	of a type and use the default value instead of the type.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>