remove CXX ABI workaround

the actual solution is you must compile zig with the same
compiler that compiled llvm, lld, and clang.

reverts 8d60ffe314

src/zig_llvm.cpp

@@ -5,15 +5,6 @@
  * See http://opensource.org/licenses/MIT
  */
 
-// This must go before all includes.
-#include "config.h"
-#if defined(ZIG_LLVM_OLD_CXX_ABI)
-#define _GLIBCXX_USE_CXX11_ABI 0
-#endif
-
-
-#include "zig_llvm.hpp"
-
 
 /*
  * The point of this file is to contain all the LLVM C++ API interaction so that:
@@ -22,6 +13,8 @@
  * 3. Prevent C++ from infecting the rest of the project.
  */
 
+#include "zig_llvm.hpp"
+
 #include <llvm/Analysis/TargetLibraryInfo.h>
 #include <llvm/Analysis/TargetTransformInfo.h>
 #include <llvm/IR/DIBuilder.h>