Add command to report unresolved references

[darichey]

Adds rust-analyzer unresolved-references which reports unresolved references. This is useful for debugging and regression testing for both rust-analyzer and project generators like Buck's rust-project.

As discussed: https://rust-lang.zulipchat.com/#narrow/stream/185405-t-compiler.2Frust-analyzer/topic/Command.20to.20report.20unresolved.20references