Order includes alphabetically and by generality.

.clang-format

@@ -94,3 +94,16 @@ SpacesInSquareBrackets: false
 Standard:        Cpp11
 TabWidth:        3
 UseTab:          Never
+
+# Order alphabetically and by generality the included header files.
+IncludeCategories:
+  - Regex:           '^"[^/]+\"'
+    Priority:        10
+  - Regex:           '^"cling/'
+    Priority:        20
+  - Regex:           '^"clang/'
+    Priority:        30
+  - Regex:           '^"llvm/'
+    Priority:        40
+  - Regex:           '^<'
+    Priority:        50