[NFC] Remove adhoc definition of MDMapT in IRMover

Summary:
The typedef was there probably because the type alias in ValueMap was private.

Test Plan:
ninja check-llvm-unit check-llvm

stack-info: PR: #118626, branch: users/artempyanykh/fast-coro-upstream/7

Author: artempyanykh

llvm/include/llvm/Linker/IRMover.h

@@ -12,6 +12,7 @@
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/DenseSet.h"
 #include "llvm/ADT/FunctionExtras.h"
+#include "llvm/IR/ValueMap.h"
 #include <functional>
 
 namespace llvm {
@@ -41,9 +42,6 @@ class IRMover {
     static bool isEqual(const StructType *LHS, const StructType *RHS);
   };
 
-  /// Type of the Metadata map in \a ValueToValueMapTy.
-  typedef DenseMap<const Metadata *, TrackingMDRef> MDMapT;
-
 public:
   class IdentifiedStructTypeSet {
     // The set of opaque types is the composite module.

llvm/lib/Linker/IRMover.cpp

@@ -373,9 +373,6 @@ class LocalValueMaterializer final : public ValueMaterializer {
   Value *materialize(Value *V) override;
 };
 
-/// Type of the Metadata map in \a ValueToValueMapTy.
-typedef DenseMap<const Metadata *, TrackingMDRef> MDMapT;
-
 /// This is responsible for keeping track of the state used for moving data
 /// from SrcM to DstM.
 class IRLinker {