Move MirPatch from rustc_middle to rustc_mir_transform.

Because it's only used in `rustc_mir_transform`. (Presumably it is
currently in `rustc_middle` because lots of other MIR-related stuff is,
but that's not a hard requirement.) And because `rustc_middle` is huge
and it's always good to make it smaller.

compiler/rustc_mir_transform/src/elaborate_drops.rs

@@ -3,7 +3,6 @@ use std::fmt;
 use rustc_abi::{FieldIdx, VariantIdx};
 use rustc_index::IndexVec;
 use rustc_index::bit_set::DenseBitSet;
-use rustc_middle::mir::patch::MirPatch;
 use rustc_middle::mir::*;
 use rustc_middle::ty::{self, TyCtxt};
 use rustc_mir_dataflow::impls::{MaybeInitializedPlaces, MaybeUninitializedPlaces};
@@ -17,6 +16,7 @@ use tracing::{debug, instrument};
 
 use crate::deref_separator::deref_finder;
 use crate::elaborate_drop::{DropElaborator, DropFlagMode, DropStyle, Unwind, elaborate_drop};
+use crate::patch::MirPatch;
 
 /// During MIR building, Drop terminators are inserted in every place where a drop may occur.
 /// However, in this phase, the presence of these terminators does not guarantee that a destructor