[AsmPrinter] Make AsmPrinter's OutStreamer member a unique_ptr.

AsmPrinter owns the OutStreamer, so an owning pointer makes sense here. Using a
reference for this is crufty.

llvm-svn: 235752
diff --git a/llvm/lib/Target/X86/X86AsmPrinter.h b/llvm/lib/Target/X86/X86AsmPrinter.h
index 9c6c683..b8aa177 100644
--- a/llvm/lib/Target/X86/X86AsmPrinter.h
+++ b/llvm/lib/Target/X86/X86AsmPrinter.h
@@ -103,7 +103,7 @@
   void EmitInstruction(const MachineInstr *MI) override;
 
   void EmitBasicBlockEnd(const MachineBasicBlock &MBB) override {
-    SMShadowTracker.emitShadowPadding(OutStreamer, getSubtargetInfo());
+    SMShadowTracker.emitShadowPadding(*OutStreamer, getSubtargetInfo());
   }
 
   bool PrintAsmOperand(const MachineInstr *MI, unsigned OpNo,