Revert "Revert r198851, "Prototype of skeleton type units for fission""

This reverts commit r198865 which reverts r198851.

ASan identified a use-of-uninitialized of the DwarfTypeUnit::Ty variable
in skeleton type units.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@198908 91177308-0d34-0410-b5e6-96231b3b80d8