[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
1 file changed