gccrs: parser: Parse external type item

gccrs: parser: Parse external type item

Add the code to parse type item declaration within an extern block.

gcc/rust/ChangeLog:

	* parse/rust-parse-impl.h (Parser::parse_external_type_item):
	Add function to parser an external type item.
	(Parser::parse_external_item):
	Add identification and parsing for external type items.
	* parse/rust-parse.h:
	Add parser_external_type_item prototype.

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