Includes module header first.

This "include module header first" is part of the llvm style guide.

This change exposes "broken" headers that were accessing undefined types, or
even using types without forward declaring them.

BUG=
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/1202253002.
diff --git a/src/IceThreading.cpp b/src/IceThreading.cpp
index 5cdaaa8..9720f77 100644
--- a/src/IceThreading.cpp
+++ b/src/IceThreading.cpp
@@ -11,9 +11,10 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "IceThreading.h"
+
 #include "IceCfg.h"
 #include "IceDefs.h"
-#include "IceThreading.h"
 
 namespace Ice {