Implement unused variable checker on HIR

[sakupan102]

This PR closes https://github.com/Rust-GCC/gccrs/issues/4086, https://github.com/Rust-GCC/gccrs/issues/4085

• [x] GCC development requires copyright assignment or the Developer's Certificate of Origin sign-off, see https://gcc.gnu.org/contribute.html or https://gcc.gnu.org/dco.html
• [x] Read contributing guidlines
• [x] make check-rust passes locally
• [x] Run clang-format

[sakupan102]

@CohenArthur I made the suggested changes based on your review. If everything looks good, I'll squash the commits.

[philberty]

There are merge conflicts here @sakupan102