[clang-format ]Extend IncludeCategories regex documentation

[WimLeflere]

Summary

Extend the Clang-Format IncludeCategories documentation by adding a link to the supported regular expression standard (POSIX).
And extenting the example with a system header regex.
bug 35041

[krasimir]

This file is automatically generated from Format.h using clang/docs/tools/dump_format_style.py. Please update.

[WimLeflere]

Added the documentation to the header file.
Also had to update dump_format_style.py because it was not loading the IncludeStyle header file.

[krasimir]

Thank you!

[WimLeflere]

Who can merge these changes?
Or does this happen automatically?

[WimLeflere]

I don't have commit access, can someone else commit this change?

docs/ClangFormatStyleOptions.rst

.. code-block:: c++
        #include <lib/main.h>
        #include "a.h"                         #include <lib/main.h>
 
 
 
 **IncludeCategories** (``std::vector<IncludeCategory>``)
   Regular expressions denoting the different ``#include`` categories
   used for ordering ``#includes``.
   
+  `POSIX extended
+  <http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap09.html>`_
+  regular expressions are supported.
+
   These regular expressions are matched against the filename of an include
   (including the <> or "") in order. The value belonging to the first
   matching regular expression is assigned and ``#includes`` are sorted first
   according to increasing category number and then alphabetically within
   each category.
 
   If none of the regular expressions match, INT_MAX is assigned as
   category. The main header for a source file automatically gets category 0.
   so that it is generally kept at the beginning of the ``#includes``
   (http://llvm.org/docs/CodingStandards.html#include-style). However, you
   can also assign negative priorities if you have certain headers that
   always need to be first.
 
   To configure this in the .clang-format file, use:
 
   .. code-block:: yaml
 
     IncludeCategories:
       - Regex:           '^"(llvm|llvm-c|clang|clang-c)/'
         Priority:        2
       - Regex:           '^(<|"(gtest|gmock|isl|json)/)'
         Priority:        3
+      - Regex:           '<[[:alnum:].]+>'
+        Priority:        4
       - Regex:           '.*'
         Priority:        1
 
 **IncludeIsMainRegex** (``std::string``)
   Specify a regular expression of suffixes that are allowed in the
   file-to-main-include mapping.
 
   When guessing whether a #include is the "main" include (to assign