DebugInfo: Move unit begin/end labels into the unit

This removes another case of spooky action at a distance (building the
same label names in multiple places creating an implicit dependency
between those places) and helps pave the way for type units.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@196617 91177308-0d34-0410-b5e6-96231b3b80d8