DebugInfo: Include the section and start-of-section label in the unit

This is a precursor to moving type units into the correct (debug_types)
section with comdat groups and full type unit headers.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@196615 91177308-0d34-0410-b5e6-96231b3b80d8