The Rust compiler is now compiled with (thin) LTO (finally) for 5-10% improvements

There was a post about this already, apparently someone has noticed the unusual perf. gains from yesterday, but didn't know where they came from :D

rustc is now compiled with (thin) LTO (PR), which resulted in very nice gains across the board, and even without any noticeable regressions!

So far it's only being done on Linux, but work is already underway to enable it on Windows and macOS too.