Use a hook to decouple `rustc_mir_transform` from `rustc_mir_build` I noticed that the only point of direct contact between the `rustc_mir_transform` and `rustc_mir_build` crates is a single `build_mir` function, which could easily be changed to a hook function instead. By making that function a hook, we can make `rustc_mir_transform` no longer have a dependency on `rustc_mir_build`, allowing them to be built/rebuilt independently. That should hopefully allow slightly more parallelism in clean builds and incremental rebuilds of the compiler.