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.

ir/irstruct.h

@@ -16,9 +16,8 @@
 #define LDC_IR_IRSTRUCT_H
 
 #include "ir/ir.h"
-
-#include <vector>
 #include <map>
+#include <vector>
 
 // DMD forward declarations
 struct StructInitializer;