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/FileSystem.h

@@ -33,11 +33,11 @@
 #include "llvm/Support/DataTypes.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/TimeValue.h"
-#include "llvm/Support/system_error.h"
 #include <ctime>
 #include <iterator>
 #include <stack>
 #include <string>
+#include <system_error>
 #include <tuple>
 #include <vector>
 
@@ -46,6 +46,7 @@
 #endif
 
 namespace llvm {
+using std::error_code;
 namespace sys {
 namespace fs {