[clang][nfc] Fix tests to avoid writing output to unreachable location (llvm#159138)

Note: this was originally done in (llvm#158698) but was reverted in the
pr/158844 (which re-applied the original rev)

Author: oontvoo

clang/test/Driver/clang_f_opts.c

@@ -52,11 +52,11 @@
 // CHECK-INTERCHANGE-LOOPS: "-floop-interchange"
 // CHECK-NO-INTERCHANGE-LOOPS: "-fno-loop-interchange"
 
-// RUN: %clang -### -S -fexperimental-loop-fusion %s 2>&1 | FileCheck -check-prefix=CHECK-FUSE-LOOPS %s
+// RUN: %clang -### -S -fexperimental-loop-fusion %s -o /dev/null 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