[llvm-cov] Try to appease a Windows bot

On a Windows bot, I see a FileCheck error where the source being matched
over no longer exists, i.e it seems like it's FileCheck'ing some stale
output:

http://lab.llvm.org:8011/builders/llvm-clang-x86_64-expensive-checks-win/builds/4747

You can see "// CHECK: [[@LINE]]|{{ +}Marker at 19:3 = 1" in the
FileCheck stderr, but that CHECK line doesn't exist.

Remove the input file to FileCheck before running the test, to try and
appease the bot.

llvm-svn: 312825