ARCMigrate: Use hash_combine in the DenseMapInfo for EditEntry

[dexonsmith]

Summary

Simplify the DenseMapInfo for EditEntry by migrating from
FoldingSetNodeID to llvm::hash_combine. Besides the cleanup, this
reduces the diff for a future patch which changes the type of one of the
fields.

There should be no real functionality change here, although I imagine
the hash value will churn since its a different hashing infrastructure.

[jansvoboda11]

LGTM.

clang/lib/ARCMigrate/ObjCMT.cpp

static inline EditEntry getEmptyKey() {
     return Entry;
   }
   static inline EditEntry getTombstoneKey() {
     EditEntry Entry;
     Entry.Offset = unsigned(-2);
     return Entry;
   }
   static unsigned getHashValue(const EditEntry& Val) {
-    llvm::FoldingSetNodeID ID;
-    ID.AddPointer(Val.File);
-    ID.AddInteger(Val.Offset);
-    ID.AddInteger(Val.RemoveLen);
-    ID.AddString(Val.Text);
-    return ID.ComputeHash();
+    return (unsigned)llvm::hash_combine(Val.File, Val.Offset, Val.RemoveLen,
+                                        Val.Text);
   }
   static bool isEqual(const EditEntry &LHS, const EditEntry &RHS) {
     return LHS.File == RHS.File &&
         LHS.Offset == RHS.Offset &&
         LHS.RemoveLen == RHS.RemoveLen &&
         LHS.Text == RHS.Text;
   }
 };