Kani compiler should include its backend version

[celinval]

Proposed change: The kani-compiler should include its version when execution includes --version --verbose.

Motivation: Cargo uses the compiler version to create a hash code for its compiler. This currently does not include the backend version, but it sounds like they are opened to change it. So this will probably be the best mechanism to implement proper caching invalidation when our users update Kani in case the new version still relies on the same version rustc version.

https://github.com/rust-lang/cargo/blob/ee755e77dae6cd419babe6f778dfff9337a4c7cf/src/cargo/core/compiler/context/compilation_files.rs#L662-L664

[celinval]

I just tried this and it looks like rustc doesn't take into account he backend passed to RunCompiler when printing its version. So this will likely need some changes to rustc as well.