FileCheck: Add a flag to allow checking empty input

Currently FileCheck errors out on empty input. This is usually the
right thing to do, but makes testing things like "this command does
not emit some error message" hard to test. This usually leads to
people using "command 2>&1 | count 0" instead, and then the bots that
use guard malloc fail a few hours later.

By adding a flag to FileCheck that allows empty inputs, we can make
tests that consist entirely of "CHECK-NOT" lines feasible.