DebugInfo: Don't put fission type units in comdat sections.

Since type units in the dwo file are handled by a debug aware tool, they
don't need to leverage the ELF comdat grouping to implement
deduplication. Avoid creating all the .group sections for these as a
space optimization.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208930 91177308-0d34-0410-b5e6-96231b3b80d8