Remove system_error.h.

This is a minimal change to remove the header. I will remove the occurrences
of "using std::error_code" in a followup patch.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210803 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Support/DataStream.cpp

@@ -18,10 +18,10 @@
 #include "llvm/ADT/Statistic.h"
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/Program.h"
-#include "llvm/Support/system_error.h"
 #include <cerrno>
 #include <cstdio>
 #include <string>
+#include <system_error>
 #if !defined(_MSC_VER) && !defined(__MINGW32__)
 #include <unistd.h>
 #else