Move MirPass to rustc_mir_transform.

Because that's now the only crate that uses it.

Moving stuff out of `rustc_middle` is always welcome.

I chose to use `impl crate::MirPass`/`impl crate::MirLint` (with
explicit `crate::`) everywhere because that's the only mention of
`MirPass`/`MirLint` used in all of these files. (Prior to this change,
`MirPass` was mostly imported via `use rustc_middle::mir::*` items.)

compiler/rustc_mir_transform/src/reveal_all.rs

@@ -6,7 +6,7 @@ use rustc_middle::ty::{self, Ty, TyCtxt};
 
 pub struct RevealAll;
 
-impl<'tcx> MirPass<'tcx> for RevealAll {
+impl<'tcx> crate::MirPass<'tcx> for RevealAll {
     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
         let param_env = tcx.param_env_reveal_all_normalized(body.source.def_id());
         RevealAllVisitor { tcx, param_env }.visit_body_preserves_cfg(body);