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/check_alignment.rs

@@ -9,7 +9,7 @@ use tracing::{debug, trace};
 
 pub struct CheckAlignment;
 
-impl<'tcx> MirPass<'tcx> for CheckAlignment {
+impl<'tcx> crate::MirPass<'tcx> for CheckAlignment {
     fn is_enabled(&self, sess: &Session) -> bool {
         // FIXME(#112480) MSVC and rustc disagree on minimum stack alignment on x86 Windows
         if sess.target.llvm_target == "i686-pc-windows-msvc" {