merging types by name when we can. We still don't guarantee type name linkage
but we do it when obviously the right thing to do. This makes LTO type names
easier to read, for example.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@146932 91177308-0d34-0410-b5e6-96231b3b80d8
