Fix -emit-codegen-only to not generate binaries

llvm-svn: 210059
diff --git a/clang/lib/CodeGen/CodeGenAction.cpp b/clang/lib/CodeGen/CodeGenAction.cpp
index d337fa7..88f02b4 100644
--- a/clang/lib/CodeGen/CodeGenAction.cpp
+++ b/clang/lib/CodeGen/CodeGenAction.cpp
@@ -568,6 +568,7 @@
   case Backend_EmitNothing:
     return nullptr;
   case Backend_EmitMCNull:
+    return new raw_null_ostream();
   case Backend_EmitObj:
     return CI.createDefaultOutputFile(true, InFile, "o");
   }