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/unreachable_enum_branching.rs

@@ -3,7 +3,6 @@
 use rustc_abi::Variants;
 use rustc_data_structures::fx::FxHashSet;
 use rustc_middle::bug;
-use rustc_middle::mir::patch::MirPatch;
 use rustc_middle::mir::{
     BasicBlock, BasicBlockData, BasicBlocks, Body, Local, Operand, Rvalue, StatementKind,
     TerminatorKind,
@@ -12,6 +11,8 @@ use rustc_middle::ty::layout::TyAndLayout;
 use rustc_middle::ty::{Ty, TyCtxt};
 use tracing::trace;
 
+use crate::patch::MirPatch;
+
 pub(super) struct UnreachableEnumBranching;
 
 fn get_discriminant_local(terminator: &TerminatorKind<'_>) -> Option<Local> {