gccrs: add unused mut lint

gccrs: add unused mut lint

gcc/rust/ChangeLog:

	* checks/lints/unused/rust-unused-checker.cc (UnusedChecker::UnusedChecker):
	Add warning for identifier pattern and field ident pattern in struct
	(UnusedChecker::visit): Add methods.
	* checks/lints/unused/rust-unused-checker.h: Same here.
	* checks/lints/unused/rust-unused-collector.cc (UnusedCollector::UnusedCollector):
	Collect unused mut variables
	(UnusedCollector::visit): Add methods.
	* checks/lints/unused/rust-unused-collector.h: Same here.
	* checks/lints/unused/rust-unused-context.cc (UnusedContext::remove_assign):
	Add methods for unused mut set.
	(UnusedContext::add_mut): Same here.
	(UnusedContext::remove_mut): Same here.
	(UnusedContext::is_mut_used): Same here.
	* checks/lints/unused/rust-unused-context.h: Same here.

gcc/testsuite/ChangeLog:

	* rust/compile/unused-mut-identifier_0.rs: New test.
	* rust/compile/unused-mut-struct-field_0.rs: New test.

Signed-off-by: Lucas Ly Ba <lucas.ly-ba@outlook.com>