Fix test.

With -### clang return 0, so we have to use FileCheck to see if there was an
error.

llvm-svn: 191923

clang/test/Driver/x86_features.c

@@ -3,4 +3,5 @@
 // Note that we filter out all but the last -m(no)sse.
 
 // Test that we don't produce an error with -mieee-fp.
-// RUN: %clang -### %s -mieee-fp -S
+// RUN: %clang -### %s -mieee-fp -S 2>&1 | FileCheck --check-prefix=IEEE %s
+// IEEE-NOT: error: unknown argument