Undo include order work-around in Regex.cpp

r327256 / 2b8b90a added this
include-order work-around since regex_impl.h's include guard conflicted
with macOS's xlocale.h which used the same macro.

Since b86c249 renamed the include
guard, that work-around is no longer needed.

Differential revision: https://reviews.llvm.org/D150127

llvm/lib/Support/Regex.cpp

@@ -14,14 +14,11 @@
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/ADT/Twine.h"
+#include "regex_impl.h"
+
 #include <cassert>
 #include <string>
 
-// Important this comes last because it defines "_REGEX_H_". At least on
-// Darwin, if included before any header that (transitively) includes
-// xlocale.h, this will cause trouble, because of missing regex-related types.
-#include "regex_impl.h"
-
 using namespace llvm;
 
 Regex::Regex() : preg(nullptr), error(REG_BADPAT) {}