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

include/llvm/MC/MCStreamer.h

@@ -658,6 +658,8 @@ public:
   virtual void EmitDwarfAdvanceFrameAddr(const MCSymbol *LastLabel,
                                          const MCSymbol *Label) {}
 
+  virtual MCSymbol *getDwarfLineTableSymbol(unsigned CUID);
+
   void EmitDwarfSetLineAddr(int64_t LineDelta, const MCSymbol *Label,
                             int PointerSize);