[NFC] make the case only require frontend

Summary:
Fixes buildbot failure like: https://lab.llvm.org/buildbot/#/builders/144/builds/2931
  caused by #99888

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Differential Revision: https://phabricator.intern.facebook.com/D60251170

Author: Chen Zheng

clang/test/Preprocessor/pragma_mc_func.c

@@ -1,4 +1,4 @@
-// RUN: not %clang --target=powerpc64-ibm-aix -ferr-pragma-mc-func-aix -c -S \
+// RUN: not %clang --target=powerpc64-ibm-aix -ferr-pragma-mc-func-aix -fsyntax-only \
 // RUN:   %s 2>&1 | FileCheck %s
 #pragma mc_func asm_barrier {"60000000"}