[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@270913 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed