[Build] Fix flag in benchmark build

Author: drexin

benchmark/cmake/modules/AddSwiftBenchmarkSuite.cmake

@@ -355,7 +355,7 @@ function (swift_benchmark_compile_archopts)
       "-c"
       "-target" "${target}"
       "-${BENCH_COMPILE_ARCHOPTS_OPT}" ${PAGE_ALIGNMENT_OPTION}
-      "-Xfrontend -enable-experimental-layout-prespecialization")
+      "-Xfrontend" "-enable-experimental-layout-prespecialization")
 
   if(SWIFT_BENCHMARK_GENERATE_DEBUG_INFO)
     list(APPEND common_options "-g")