[llc] -filetype=null: don't create .null if -o is not specified

The new behavior is consistent with llvm-mc -filetype=null.

Author: MaskRay

llvm/test/CodeGen/X86/null-streamer.ll

@@ -1,7 +1,8 @@
 ; Check the MCNullStreamer operates correctly, at least on a minimal test case.
 ;
+; RUN: rm -f %t
 ; RUN: llc -filetype=null -o %t -mtriple=i686-- %s
-; RUN: llc -filetype=null -o %t -mtriple=i686-cygwin %s
+; RUN: ls %t
 
 source_filename = "test/CodeGen/X86/null-streamer.ll"
 

llvm/tools/llc/llc.cpp

@@ -224,7 +224,7 @@ static std::unique_ptr<ToolOutputFile> GetOutputStream(const char *TargetName,
           OutputFilename += ".o";
         break;
       case CGFT_Null:
-        OutputFilename += ".null";
+        OutputFilename = "-";
         break;
       }
     }