DebugInfo: Avoid creating unnecessary/empty line tables and remove the special case of '0' in DwarfCompileUnit::initStmtList by just always using a label difference

This moves one case of raw text checking down into the MCStreamer
interfaces in the form of a virtual function, even if we ultimately end
up consolidating on the one-or-many line tables issue one day, this is
nicer in the interim. This just generally streamlines a bunch of use
cases into a common code path.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205287 91177308-0d34-0410-b5e6-96231b3b80d8