[LibFuzzer] Emit error if LLVM_USE_SANITIZER is not correctly set. Previously CMake would successfully configure and compile (with warnings about ``-fsanitize-coverage=...`` being unused) but the tests LibFuzzer tests would fail. Differential Revision: http://reviews.llvm.org/D20662 llvm-svn: 270913 GitOrigin-RevId: c752a2642895e496ab4ded99deda7e8cec4bbcab