test/Modules: Use FileCheck's -allow-empty instead of "count 0"

Piping stderr into "count 0" in tests doesn't work - things like guard
malloc write to stderr and mess up the count. This comes up all the
time, so I've added a feature to FileCheck to fix it this time.

Fixes test failures caused by r215046 under guard malloc.

llvm-svn: 215129