[llvm-dwp] Deduplicate type units

It's O(N^2) because it does a simple walk through the existing types to
find duplicates, but that will be fixed in a follow-up commit to use a
mapping data structure of some kind.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@255482 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed