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.

llvm-svn: 210803
diff --git a/llvm/lib/Support/Program.cpp b/llvm/lib/Support/Program.cpp
index 913c099..eb700e3 100644
--- a/llvm/lib/Support/Program.cpp
+++ b/llvm/lib/Support/Program.cpp
@@ -13,7 +13,7 @@
 
 #include "llvm/Support/Program.h"
 #include "llvm/Config/config.h"
-#include "llvm/Support/system_error.h"
+#include <system_error>
 using namespace llvm;
 using namespace sys;