llvm6: Different return value for writeArchive

Updated in llvm-mirror/llvm@203c90ba this function now just returns an `Error`,
so this updates the C++ bindings accordingly

Author: alexcrichton

src/rustllvm/ArchiveWrapper.cpp

@@ -232,9 +232,16 @@ LLVMRustWriteArchive(char *Dst, size_t NumMembers,
       Members.push_back(std::move(*MOrErr));
     }
   }
-  auto Pair = writeArchive(Dst, Members, WriteSymbtab, Kind, true, false);
-  if (!Pair.second)
+  auto Result = writeArchive(Dst, Members, WriteSymbtab, Kind, true, false);
+#if LLVM_VERSION_GE(6, 0)
+  if (!Result)
     return LLVMRustResult::Success;
-  LLVMRustSetLastError(Pair.second.message().c_str());
+  LLVMRustSetLastError(toString(std::move(Result)).c_str());
+#else
+  if (!Result.second)
+    return LLVMRustResult::Success;
+  LLVMRustSetLastError(Result.second.message().c_str());
+#endif
+
   return LLVMRustResult::Failure;
 }