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

include/llvm/Support/LockFileManager.h

@@ -12,11 +12,11 @@
 #include "llvm/ADT/Optional.h"
 #include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/StringRef.h"
-#include "llvm/Support/system_error.h"
+#include <system_error>
 #include <utility> // for std::pair
 
 namespace llvm {
-
+using std::error_code;
 /// \brief Class that manages the creation of a lock file to aid
 /// implicit coordination between different processes.
 ///