The `.clang-format` configuration needs no changes to make the
version 15 format close to what version 6.0 produced before.

Issue: #24315

Fixes: #20666

The rule Regex: '^<' was not applied to this header because a more specific one applies: Regex: '^(<|")Qt?[A-Z]' used for Qt headers (rules are not case-sensitive). So adding a specific rule for this header before the Qt rule is required.

By using clang-format directives 'IncludeBlocks' and 'IncludeCategories'
headers are regrouped by categories:
1. "cmConfigure.h" which must be first
2. header matching current implementation file
3. standard language headers
4. CMake replacement ones
5. external  headers (i.e. #include directive with '<' and '>')
6. Qt headers
7. cmsys/* headers
8. cm_* headers
9. CMake headers (i.e. starting with 'cm[A-Z]')
10. all other headers

Fixes: #19674

Update `.clang-format` with configuration to make the 6.0 format as
close as possible to what 3.8 produced before.  Then revise the style:

* Indent preprocessor directives (a feature new since 3.8)
* Add a newline and indentation before inheritance `:` and `,`

Rename the Git attribute identifying the format to include the
clang-format version number: `format.clang-format-6.0`.  This will aid
external infrastructure in knowing what version of the tool to run.

Versions of clang-format newer than 3.8 produce different output.
Our source code guide says that version 3.8 exactly must be used.
Update the comment in our `.clang-format` config file accordingly.
Add a check to our formatting script to verify the version.
Also update its tool search to look for the explicitly-named
3.8 variant first.

After discussion among developers we settled on the Mozilla style
with a few tweaks:

* Do not align operator arguments.

* Do not always break after a function return type.

* Limit to 79 columns instead of 80 to fit edge cases
  better in 80-column terminals as CMake has always done.

* Format for C++98 instead of C++11 because CMake is written
  in the former language.

Co-Author: Daniel Pfeifer <daniel@pfeifer-mail.de>