[CMake] Fix USE_LLVM_SANITIZER configuration for out-of-tree builds.

Summary:
r291918 changed `HandleLLVMOptions.cmake` to add `-fsanitize-blacklist=<llvm-file>` when `LLVM_USE_SANITIZER=Undefined` is specified. This breaks out-of-tree users of `LLVM_USE_SANITIZER` since that file is not present.

This patch fixes the issue by checking if the file exists first.

Reviewers: mgorny, bogner, vitalybuka, krasin

Reviewed By: krasin

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D29686

llvm-svn: 294367