fuzzing: use llvm source code instrumentation

fuzzing/afl.sh

@@ -5,5 +5,5 @@ cd afl-build || exit 1
 #cleanup
 rm -r -- *
 
-CC=afl-gcc cmake ../.. -DENABLE_FUZZING=On -DENABLE_SANITIZERS=On -DBUILD_SHARED_LIBS=Off
+CC=afl-clang-fast cmake ../.. -DENABLE_FUZZING=On -DENABLE_SANITIZERS=On -DBUILD_SHARED_LIBS=Off
 make afl