[clang][nfc] Fix tests to avoid writing output to unreachable location, which might cause them to fail (#158698)

[clang][nfc] Fix tests to avoid writing output to unreachable location,
which might cause them to fail

clang/test/Driver/clang_f_opts.c

diff --git a/clang/test/Driver/clang_f_opts.c b/clang/test/Driver/clang_f_opts.c
index eb3994d..581b031 100644
--- a/clang/test/Driver/clang_f_opts.c
+++ b/clang/test/Driver/clang_f_opts.c
@@ -55,8 +55,8 @@
 // RUN: %clang -### -S -fexperimental-loop-fusion %s 2>&1 | FileCheck -check-prefix=CHECK-FUSE-LOOPS %s
 // CHECK-FUSE-LOOPS: "-fexperimental-loop-fusion"
 //
-// RUN: %clang -c -fexperimental-loop-fusion -mllvm -print-pipeline-passes -O3 %s 2>&1 | FileCheck --check-prefixes=LOOP-FUSION-ON %s
-// RUN: %clang -c -mllvm -print-pipeline-passes -O3 %s 2>&1 | FileCheck --check-prefixes=LOOP-FUSION-OFF %s
+// RUN: %clang -c -fexperimental-loop-fusion -mllvm -print-pipeline-passes -O3 %s -o /dev/null 2>&1 | FileCheck --check-prefixes=LOOP-FUSION-ON %s
+// RUN: %clang -c -mllvm -print-pipeline-passes -O3 %s -o /dev/null 2>&1 | FileCheck --check-prefixes=LOOP-FUSION-OFF %s
 
 // LOOP-FUSION-ON: loop-fusion
 // LOOP-FUSION-OFF-NOT: loop-fusion