gccrs: Add support for parsing empty tuple patterns.

gcc/rust/ChangeLog:

	* parse/rust-parse-impl.h
	(Parser::parse_grouped_or_tuple_pattern): Add support for empty tuple patterns.

gcc/testsuite/ChangeLog:

	* rust/compile/parse_empty_tuple_pattern.rs: New test.

Signed-off-by: Owen Avery <powerboat9.gamer@gmail.com>