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/promote_consts.rs

@@ -41,7 +41,7 @@ pub struct PromoteTemps<'tcx> {
     pub promoted_fragments: Cell<IndexVec<Promoted, Body<'tcx>>>,
 }
 
-impl<'tcx> MirPass<'tcx> for PromoteTemps<'tcx> {
+impl<'tcx> crate::MirPass<'tcx> for PromoteTemps<'tcx> {
     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
         // There's not really any point in promoting errorful MIR.
         //