gccrs: add unused-assignments lint

gccrs: add unused-assignments lint

gcc/rust/ChangeLog:

	* checks/lints/unused-var/rust-unused-var-checker.cc (UnusedVarChecker):
	Implement unused assignments warning.
	(UnusedVarChecker::go): Remove unique pointer unused var context.
	(UnusedVarChecker::visit): Visit AssignExpr in HIR default visitor.
	* checks/lints/unused-var/rust-unused-var-checker.h: Add visit method.
	* checks/lints/unused-var/rust-unused-var-collector.cc (UnusedVarCollector):
	Collect warnings for assignments.
	(UnusedVarCollector::visit): Visit AssignExpr in HIR default visitor.
	* checks/lints/unused-var/rust-unused-var-collector.h: Add visit method.
	* checks/lints/unused-var/rust-unused-var-context.cc (UnusedVarContext::add_assign):
	Add assignment in map.
	(UnusedVarContext::remove_assign): Remove assignment in map.
	(UnusedVarContext::is_variable_assigned): Check if a variable is assigned.
	* checks/lints/unused-var/rust-unused-var-context.h: Add a map to stock assignments.

gcc/testsuite/ChangeLog:

	* rust/compile/issue-4260_0.rs: New test.

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