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/irfunction.h

@@ -19,10 +19,9 @@
 #include "gen/llvm.h"
 #include "ir/ir.h"
 #include "ir/irlandingpad.h"
-
-#include <vector>
-#include <stack>
 #include <map>
+#include <stack>
+#include <vector>
 
 struct Statement;
 struct EnclosingHandler;