gccrs: implement unused variable checker on HIR.

gccrs: implement unused variable checker on HIR.

This change moves the unused variable checker from the type resolver
to HIR. We can now use the HIR Default Visitor, and it will be much more
easier to implement other unused lints with this change.

gcc/rust/ChangeLog:

	* Make-lang.in: Add new files rules in Makefile.
	* lang.opt: Add new flag.
	* rust-session-manager.cc (Session::compile_crate): Execute new variable checker.
	* checks/lints/unused-var/rust-unused-var-checker.cc (UnusedVarChecker): Implement unused
	variable checker.
	* checks/lints/unused-var/rust-unused-var-checker.h (UnusedVarChecker): Implement unused
	variable checker.
	* checks/lints/unused-var/rust-unused-var-collector.cc (UnusedVarCollector): Implement
	unused variable collector.
	* checks/lints/unused-var/rust-unused-var-collector.h (UnusedVarCollector): Implement
	unused variable collector.
	* checks/lints/unused-var/rust-unused-var-context.cc (UnusedVarContext): Implement
	unused variable context.
	* checks/lints/unused-var/rust-unused-var-context.h (UnusedVarContext): Implement unused
	variable context.

gcc/testsuite/ChangeLog:

	* rust/compile/static_item_0.rs: New test.
	* rust/compile/template_function_0.rs: New test.

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