Implement unused variable checker on HIR

[lucasly-ba]

This is the prolongation of PR #4055, since it might be outdated.

[dkm]

gcc/rust/ChangeLog:

	* Make-lang.in: New file.

This is misleading. "New file" is idiomatic for "this is a new file". You probably meant "I've added new file rules in the makefile". Can you rephrase that and make it less confusing? Thanks!

[lucasly-ba]

minor style nit.. i would be interested how bad are the regressions when you force this to be the only unused variable checker?

With this command : grep -r "unused name" ./ | wc -l It gives us 211 lines in the testsuite