Refactor node description in AST dump

Rename the functions and split internal comments from node description.

gcc/rust/ChangeLog:

	* ast/rust-ast-collector.cc (TokenCollector::comment): Use comment kind
	(TokenCollector::begin_internal_comment): Rename from this...
	(TokenCollector::begin_describe_node): ... to this.
	(TokenCollector::end_internal_comment): Rename from this...
	(TokenCollector::end_describe_node): ... to this.
	(TokenCollector::internal_comment): Rename from this...
	(TokenCollector::describe_node): ... to this.
	(TokenCollector::visit): Change name in function calls.
	(TokenCollector::visit_closure_common): Likewise.
	(TokenCollector::visit_loop_common): Likewise.
	* ast/rust-ast-collector.h: Adapt function prototypes, add a new
	collect item kind for node description.

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

Author: P-E-P