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/large_enums.rs

@@ -27,7 +27,7 @@ pub struct EnumSizeOpt {
     pub(crate) discrepancy: u64,
 }
 
-impl<'tcx> MirPass<'tcx> for EnumSizeOpt {
+impl<'tcx> crate::MirPass<'tcx> for EnumSizeOpt {
     fn is_enabled(&self, sess: &Session) -> bool {
         // There are some differences in behavior on wasm and ARM that are not properly
         // understood, so we conservatively treat this optimization as unsound: