Merge pull request #6625 from tautschnig/bugfixes/_class-header

clang-format header include grouping: _class.h has top priority

Author: kroening

.clang-format

@@ -49,6 +49,8 @@ PointerAlignment: Right
 ReflowComments: 'false'
 IncludeBlocks: Regroup
 IncludeCategories:
+  - Regex: '_class.h(>|")$'
+    Priority: 0
   - Regex: '<util/'
     Priority: 1
   - Regex: '<goto-programs/'