Sort includes according to style guidelines:

1. Main include corresponding to .cpp file, if any.
 2. DMD and LDC includes.
 3. LLVM includes.
 4. System includes.

Also updated a few include guards to match the default format.

gen/cl_helpers.cpp

@@ -8,15 +8,13 @@
 //===----------------------------------------------------------------------===//
 
 #include "gen/cl_helpers.h"
-
 #include "mars.h"
-#include "root.h"
 #include "rmem.h"
-
-#include <cctype>       // isupper, tolower
+#include "root.h"
 #include <algorithm>
-#include <utility>
+#include <cctype>       // isupper, tolower
 #include <stdarg.h>
+#include <utility>
 
 namespace opts {