[Debug Info] unique MDNodes in the enum types of each compile unit.

The enum types array by design contains pointers to MDNodes rather than DIRefs.
Unique them when handling the enum types in DwarfDebug.

rdar://17628609


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214139 91177308-0d34-0410-b5e6-96231b3b80d8