[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/MSP430/MSP430AsmPrinter.cpp b/llvm/lib/Target/MSP430/MSP430AsmPrinter.cpp
index fb7823e..a99c9a3 100644
--- a/llvm/lib/Target/MSP430/MSP430AsmPrinter.cpp
+++ b/llvm/lib/Target/MSP430/MSP430AsmPrinter.cpp
@@ -152,7 +152,7 @@
 
   MCInst TmpInst;
   MCInstLowering.Lower(MI, TmpInst);
-  EmitToStreamer(OutStreamer, TmpInst);
+  EmitToStreamer(*OutStreamer, TmpInst);
 }
 
 // Force static initialization.