Fix C++ code compilation on LLVM 4.0

Author: yuyichao

src/jitlayers.cpp

@@ -43,7 +43,11 @@ namespace llvm {
     extern Pass *createLowerSimdLoopPass();
 }
 
-#include <llvm/Bitcode/ReaderWriter.h>
+#if JL_LLVM_VERSION >= 40000
+#  include <llvm/Bitcode/BitcodeWriter.h>
+#else
+#  include <llvm/Bitcode/ReaderWriter.h>
+#endif
 #if JL_LLVM_VERSION >= 30500
 #include <llvm/Bitcode/BitcodeWriterPass.h>
 #endif