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/ctfe_limit.rs

@@ -8,11 +8,9 @@ use rustc_middle::mir::{
 use rustc_middle::ty::TyCtxt;
 use tracing::instrument;
 
-use crate::MirPass;
-
 pub struct CtfeLimit;
 
-impl<'tcx> MirPass<'tcx> for CtfeLimit {
+impl<'tcx> crate::MirPass<'tcx> for CtfeLimit {
     #[instrument(skip(self, _tcx, body))]
     fn run_pass(&self, _tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
         let doms = body.basic_blocks.dominators();