DebugInfo: Remove unused start/end labels for the debug_abbrevs section

Since we always emit only one abbrevation section (shared by all the
compilation units in this module) there's no need for a separate label
at the start of each one (and we weren't using the CU ID anyway, so
there really was only one label). Use the section label instead and drop
the wholely unused debug_abbrev_end label.

llvm-svn: 196394