Revert "[test] Default clang/test to FileCheck --allow-unused-prefixes=false"
This reverts commit 80f539526eec31f03aadd96753648686312b1ad1.
Many test failures on mac: http://45.33.8.238/macm1/2772/summary.html
One on win: http://45.33.8.238/win/32442/summary.html