gccrs: change unused lint files name to unused

gcc/rust/ChangeLog:

	* Make-lang.in: Compile the right files.
	* checks/lints/unused-var/rust-unused-var-checker.cc: Move to...
	* checks/lints/unused/rust-unused-checker.cc: ...here.
	* checks/lints/unused-var/rust-unused-var-checker.h: Move to...
	* checks/lints/unused/rust-unused-checker.h: ...here.
	* checks/lints/unused-var/rust-unused-var-collector.cc: Move to...
	* checks/lints/unused/rust-unused-collector.cc: ...here.
	* checks/lints/unused-var/rust-unused-var-collector.h: Move to...
	* checks/lints/unused/rust-unused-collector.h: ...here.
	* checks/lints/unused-var/rust-unused-var-context.cc: Move to...
	* checks/lints/unused/rust-unused-context.cc: ...here.
	* checks/lints/unused-var/rust-unused-var-context.h: Move to...
	* checks/lints/unused/rust-unused-context.h: ...here.
	* rust-session-manager.cc (Session::compile_crate): Call the right method.

Signed-off-by: Lucas Ly Ba <[email protected]>

Author: lucasly-ba