[LibFuzzer] Emit error during configuration if LLVM_USE_SANITIZER is not correctly set.

[delcypher]

Summary

[LibFuzzer] Emit error during configuration 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.

This patch is dependent on http://reviews.llvm.org/D20558

[kcc]

I would rephrase it to something like this:
Currently libFuzzer and its tests require LLVM_USE_SANITIZER=Address and LLVM_USE_SANITIZE_COVERAGE=YES to be set.
Ok to commit with such or similar phrasing.

[delcypher]

@kcc : I can change the phrasing if you like but isn't mentioning LLVM_USE_SANITIZE_COVERAGE=YES redundant because the user will only see this message if they set LLVM_USE_SANITIZE_COVERAGE=YES?

[kcc]

In D20662#441359, @delcypher wrote:

@kcc : I can change the phrasing if you like but isn't mentioning LLVM_USE_SANITIZE_COVERAGE=YES redundant because the user will only see this message if they set LLVM_USE_SANITIZE_COVERAGE=YES?

It might be redundant but it's also correct and may help avoid some confusion.

lib/Fuzzer/CMakeLists.txt

 set(LIBFUZZER_FLAGS_BASE "${CMAKE_CXX_FLAGS}")
 # Disable the coverage and sanitizer instrumentation for the fuzzer itself.
 set(CMAKE_CXX_FLAGS "${LIBFUZZER_FLAGS_BASE} -fno-sanitize=all -fno-sanitize-coverage=edge,trace-cmp,indirect-calls,8bit-counters -Werror")
 if( LLVM_USE_SANITIZE_COVERAGE )
+  if(NOT "${LLVM_USE_SANITIZER}" STREQUAL "Address")
+    message(FATAL_ERROR
+      "LLVM_USE_SANITIZER must be set to Address"
+      " to build LibFuzzer"
+      )
+  endif()
   add_library(LLVMFuzzerNoMainObjects OBJECT
     FuzzerCrossOver.cpp
     FuzzerTraceState.cpp
     FuzzerDriver.cpp
     FuzzerIO.cpp
     FuzzerLoop.cpp
     FuzzerMutate.cpp
     FuzzerSHA1.cpp