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/Program.h

@@ -16,9 +16,10 @@
 
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/Support/Path.h"
-#include "llvm/Support/system_error.h"
+#include <system_error>
 
 namespace llvm {
+using std::error_code;
 namespace sys {
 
   /// This is the OS-specific separator for PATH like environment variables: