Sort includes using clang-format

In current version of clang-format, this leaves groupings alone and just
sorts within the group.

For example, given a list of includes like:

```
include "boolbv.h"

include <util/std_types.h>
include <util/std_expr.h>
include <util/arith_tools.h>
include <util/base_type.h>
include <util/pointer_offset_size.h>

include <util/c_types.h>

```

These will be sorted to:

```
include "boolbv.h"

include <util/arith_tools.h>
include <util/base_type.h>
include <util/pointer_offset_size.h>
include <util/std_expr.h>
include <util/std_types.h>

include <util/c_types.h>
```

Note the "groups" are left untouched, the ordering occurs within the
groups

.clang-format

@@ -84,7 +84,7 @@ NamespaceIndentation: None
 PenaltyBreakString: 10000
 PointerAlignment: Right
 ReflowComments: 'false'
-SortIncludes: 'false'
+SortIncludes: 'true'
 SpaceAfterCStyleCast: 'false'
 SpaceBeforeAssignmentOperators: 'true'
 SpaceBeforeParens: Never