AFLplusplus: Build error with old LLVM versions

Look at the xenial-arm64 Travis build:

afl-llvm-pass.so.cc:39:10: fatal error: 'llvm/IR/DebugInfo.h' file not found

#include "llvm/IR/DebugInfo.h"

Seems that with old LLVM the header is `llvm/DebugInfo.h".

Note that this header is not included in original AFL, so probably we broken the compatibility woth LLVM < 3.7. I say 3.7 cause is the first version with the header inside IR that I’ve found googling.

This needs more investigation, as a quick fix I added

#include <llvm/Config/llvm-config.h>

#if (LLVM_VERSION_MAJOR <= 3) && (LLVM_VERSION_MINOR < 7)
#include <llvm/DebugInfo.h>
#else
#include <llvm/IR/DebugInfo.h>
#endif

But I’m not sure about the LLVM version.