Change optional to expected for parse_loop_label

Change optional to expected for parse_loop_label

gcc/rust/ChangeLog:

	* parse/rust-parse-impl.h (Parser::parse_loop_label): Change function
	return type to expected.
	(Parser::parse_labelled_loop_expr): Adapt call location to new return
	type.
	* parse/rust-parse.h (enum class): Update function prototype.

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