Revert "Store TypeUnits in a SmallVector<DWARFUnitSection> instead of a single DWARFUnitSection."

This reverts commit r218513.

Buildbots using libstdc++ issue an error when trying to copy
SmallVector<std::unique_ptr<>>. Revert the commit until we have a fix.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218514 91177308-0d34-0410-b5e6-96231b3b80d8