[CFI] Force LLVM to die if the implicit blacklist files cannot be found.

Currently LLVM CFI tries to use an implicit blacklist file, currently
in /usr/lib64/clang/<version>/share. If the file is not there, LLVM
happily continues, which causes CFI to add checks to files/functions
that are known to fail, generating binaries that fail. This CL causes
LLVM to die (I hope) if it can't find these implicit blacklist files.

Patch by Caroline Tice!

Differential Revision: https://reviews.llvm.org/D46403

llvm-svn: 331674