[MC] Add interface to finish pending labels.

[JDevlieghere]

Summary

When manually finishing the object writer in dsymutil, it's possible
that there are pending labels that haven't been resolved. This results
in an assertion when the assembler tries to fixup a label that doesn't
have an address yet.

[JDevlieghere]

• Add comment

[JDevlieghere]

• Fix call in MachOUtils.cpp

[thegameg]

LGTM

llvm/trunk/include/llvm/MC/MCObjectStreamer.h

protected:
   /// they get assigned, either to F if possible or to a new data fragment.
   /// Optionally, it is also possible to provide an offset \p FOffset, which
   /// will be used as a symbol offset within the fragment.
   void flushPendingLabels(MCFragment *F, uint64_t FOffset = 0);
 
 public:
   void visitUsedSymbol(const MCSymbol &Sym) override;
 
+  /// Create a dummy fragment to assign any pending labels.
+  void flushPendingLabels() { flushPendingLabels(nullptr); }
+
   MCAssembler &getAssembler() { return *Assembler; }
   MCAssembler *getAssemblerPtr() override;
   /// \name MCStreamer Interface
   /// @{
 
   void EmitLabel(MCSymbol *Symbol, SMLoc Loc = SMLoc()) override;
   virtual void EmitLabel(MCSymbol *Symbol, SMLoc Loc, MCFragment *F);
   void EmitAssignment(MCSymbol *Symbol, const MCExpr *Value) override;

llvm/trunk/lib/MC/MCObjectStreamer.cpp

void MCObjectStreamer::FinishImpl() {
 
   // If we are generating dwarf for assembly source files dump out the sections.
   if (getContext().getGenDwarfForAssembly())
     MCGenDwarfInfo::Emit(this);
 
   // Dump out the dwarf file & directory tables and line tables.
   MCDwarfLineTable::Emit(this, getAssembler().getDWARFLinetableParams());
 
-  flushPendingLabels(nullptr);
+  flushPendingLabels();
   getAssembler().Finish();
 }

llvm/trunk/tools/dsymutil/MachOUtils.cpp

 // Stream a dSYM companion binary file corresponding to the binary referenced
 // by \a DM to \a OutFile. The passed \a MS MCStreamer is setup to write to
 // \a OutFile and it must be using a MachObjectWriter object to do so.
 bool generateDsymCompanion(const DebugMap &DM, MCStreamer &MS,
                            raw_fd_ostream &OutFile) {
   auto &ObjectStreamer = static_cast<MCObjectStreamer &>(MS);
   MCAssembler &MCAsm = ObjectStreamer.getAssembler();
   auto &Writer = static_cast<MachObjectWriter &>(MCAsm.getWriter());
-  MCAsmLayout Layout(MCAsm);
 
+  // Layout but don't emit.
+  ObjectStreamer.flushPendingLabels();
+  MCAsmLayout Layout(MCAsm);
   MCAsm.layout(Layout);
 
   BinaryHolder InputBinaryHolder(false);
 
   auto ObjectEntry = InputBinaryHolder.getObjectEntry(DM.getBinaryPath());
   if (!ObjectEntry) {
     auto Err = ObjectEntry.takeError();
     return error(Twine("opening ") + DM.getBinaryPath() + ": " +