Update discriminator assignment algorithm in clang assembler.

[danielcdh]

Summary

The clang assembler assumes that the discriminator remains the same when there is source line change. The correct behavior is that when there is line change, discriminator will automatically reset to 0.

[echristo]

LGTM.

-eric

lib/MC/MCDwarf.cpp

EmitDwarfLineTable(MCObjectStreamer *MCOS, MCSection *Section,
   unsigned Discriminator = 0;
   MCSymbol *LastLabel = nullptr;
 
   // Loop through each MCDwarfLineEntry and encode the dwarf line number table.
   for (auto it = LineEntries.begin(),
             ie = LineEntries.end();
        it != ie; ++it) {
 
+    int64_t LineDelta = static_cast<int64_t>(it->getLine()) - LastLine;
+
+    // Discriminator will be cleared if there is line change.
+    if (LineDelta != 0)
+      Discriminator = 0;
+
     if (FileNum != it->getFileNum()) {
       FileNum = it->getFileNum();
       MCOS->EmitIntValue(dwarf::DW_LNS_set_file, 1);
       MCOS->EmitULEB128IntValue(FileNum);
     }
     if (Column != it->getColumn()) {
       Column = it->getColumn();
       MCOS->EmitIntValue(dwarf::DW_LNS_set_column, 1);
for (auto it = LineEntries.begin(),
     }
     if (it->getFlags() & DWARF2_FLAG_BASIC_BLOCK)
       MCOS->EmitIntValue(dwarf::DW_LNS_set_basic_block, 1);
     if (it->getFlags() & DWARF2_FLAG_PROLOGUE_END)
       MCOS->EmitIntValue(dwarf::DW_LNS_set_prologue_end, 1);
     if (it->getFlags() & DWARF2_FLAG_EPILOGUE_BEGIN)
       MCOS->EmitIntValue(dwarf::DW_LNS_set_epilogue_begin, 1);
 
-    int64_t LineDelta = static_cast<int64_t>(it->getLine()) - LastLine;
     MCSymbol *Label = it->getLabel();
 
     // At this point we want to emit/create the sequence to encode the delta in
     // line numbers and the increment of the address from the previous Label
     // and the current Label.
     const MCAsmInfo *asmInfo = MCOS->getContext().getAsmInfo();
     MCOS->EmitDwarfAdvanceLineAddr(LineDelta, LastLabel, Label,
                                    asmInfo->getPointerSize());