Codify grouping and order of includes

clang-format supports describing groups of includes via regular
expressions and assigning priorities to them. The include file
corresponding to an implementation will always have priority 0. Other
priorities are now set in the clang-format configuration. Includes not
specified will be auto-assigned INT_MAX as priority.

Author: tautschnig

.clang-format

@@ -47,7 +47,18 @@ NamespaceIndentation: None
 PenaltyBreakString: 10000
 PointerAlignment: Right
 ReflowComments: 'false'
-SortIncludes: 'true'
+IncludeBlocks: Regroup
+IncludeCategories:
+  - Regex: '<util/'
+    Priority: 1
+  - Regex: '<goto-programs/'
+    Priority: 2
+  - Regex: '<.+/.+>'
+    Priority: 3
+  - Regex: '^"[^/]+"$'
+    Priority: 4
+  - Regex: '<[^/]+>'
+    Priority: 5
 SpaceAfterCStyleCast: 'false'
 SpaceBeforeAssignmentOperators: 'true'
 SpaceBeforeParens: Never

CODING_STANDARD.md

@@ -148,7 +148,7 @@ Formatting is enforced using clang-format. For more information about this, see
 - The corresponding header for a given source file should always be the *first*
   include in the source file. For example, given `foo.h` and `foo.cpp`, the
   line `#include "foo.h"` should precede all other include statements in
-  `foo.cpp`.
+  `foo.cpp`. clang-format will enforce this.
 - Use the C++ versions of C headers (e.g. `cmath` instead of `math.h`).
   Some of the C headers use macros instead of functions which can have
   unexpected consequences.