gccrs: refactor unused var lint

gccrs: refactor unused var lint

gcc/rust/ChangeLog:

	* checks/lints/unused-var/rust-unused-var-checker.cc (UnusedVarChecker::visit):
	Change unused name warning to unused variable warning.
	* checks/lints/unused-var/rust-unused-var-collector.cc (UnusedVarCollector::visit):
	Remove useless methods.
	* checks/lints/unused-var/rust-unused-var-collector.h: Same here.
	* checks/lints/unused-var/rust-unused-var-context.cc (UnusedVarContext::add_variable):
	Add used variables to set.
	(UnusedVarContext::mark_used): Remove method.
	(UnusedVarContext::is_variable_used):
	Check if the set contains the hir id linked to a variable.
	(UnusedVarContext::as_string): Refactor method for new set.
	* checks/lints/unused-var/rust-unused-var-context.h: Refactor methods.
	* lang.opt: Change description for unused check flag.

gcc/testsuite/ChangeLog:

	* rust/compile/static_item_0.rs: Modify warning output.
	* rust/compile/template_function_0.rs: Modify warning output.

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