Exploring Crate Graph Build Times with `cargo build -Ztimings`

Thanks for the awesome work on this!

I noticed on one of my side projects that the final link steps was taking almost 20 seconds, with only 4 seconds spent on codegen. Do you have any idea why that slowdown could be there? It makes it very frustrating to write integration tests and small incremental changes.

Here's a screenshot of the build times:

Project on github: https://github.com/jyn514/rcc