Debug info support

[bcardosolopes]

LLVM lowering doesn't yet include debug information, nor CIRGen currently emits it in any form. Most of the missing parts are tracked by functions in UnimplementedFeatures::*.

This issue can be used to track the overall work needed, but specific tasks should be tracked in their own issues. There are also multiple ways to go about implementing this, so it's probably good to start some discussion here before putting up a PR.