llvm-objdump: Pass DiceTableEntry by reference

DiceTableEntry is 24 bytes on my machine, it's probably better to pass
them by reference.

This fixes PR21464.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221247 91177308-0d34-0410-b5e6-96231b3b80d8