[dotest] Make a missing FileCheck binary a warning, not an error

This allows bots which haven't updated to pass in --filecheck to
dotest.py to run more tests. FileCheck-dependent tests will continue to
fail.

Differential Revision: https://reviews.llvm.org/D53175

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@344401 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed