DebugInfo: Implement debug_line.dwo for file names used in type units during -gsplit-dwarf

This removes an attribute (and more importantly, a relocation) from
skeleton type units and removes some unnecessary file names from the
debug_line section that remains in the .o (and linked executable) file.

There's still a few places we could shave off some more space here:

* use compilation dir of the underlying compilation unit (since all the
  type units share that compilation dir - though this would be more
  complicated in LTO cases where they don't (keep a map of compilation
  dir->line table header?))

* Remove some of the unnecessary header fields from the line table since
  they're not needed in this situation (about 12 bytes per table).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204099 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/AsmPrinter/DwarfDebug.h

@@ -27,6 +27,7 @@
 #include "llvm/IR/DebugInfo.h"
 #include "llvm/IR/DebugLoc.h"
 #include "llvm/MC/MachineLocation.h"
+#include "llvm/MC/MCDwarf.h"
 #include "llvm/Support/Allocator.h"
 
 namespace llvm {
@@ -486,6 +487,10 @@ class DwarfDebug : public AsmPrinterHandler {
   // Holder for the skeleton information.
   DwarfFile SkeletonHolder;
 
+  // Store file names for type units under fission in a line table header that
+  // will be emitted into debug_line.dwo.
+  MCDwarfLineTableHeader SplitTypeUnitFileTable;
+
   void addScopeVariable(LexicalScope *LS, DbgVariable *Var);
 
   const SmallVectorImpl<DwarfUnit *> &getUnits() {
@@ -621,6 +626,9 @@ class DwarfDebug : public AsmPrinterHandler {
   /// \brief Emit the debug abbrev dwo section.
   void emitDebugAbbrevDWO();
 
+  /// \brief Emit the debug line dwo section.
+  void emitDebugLineDWO();
+
   /// \brief Emit the debug str dwo section.
   void emitDebugStrDWO();