Rework code to call one function when add comment

When we want to add an internal comment we know call one function that
wrap the code instead of calling two function, one at the beginning one
at the end.

gcc/rust/ChangeLog:

	* ast/rust-ast-collector.cc (TokenCollector::internal_comment):
		Wrapper function to add both  comment.
	(TokenCollector::visit): call of the wrapper function.
	(TokenCollector::visit_closure_common): Same.
	(TokenCollector::visit_loop_common): Same.
	* ast/rust-ast-collector.h: Prototype of the wrapper function

Signed-off-by: Benjamin Thos <[email protected]>