(Hopefully an) Improvement: Separate eclass variables into extern (C) and constexpr (C++) variables

[sandro-elsweijer]

Closes #1943

Describe your changes here:
Possibly the worst PR I ever made.

To be able to use the eclass variables in constexpr contexts the variables themselves have to be constexpr.
But since C does not understand constexpr and since we still have C files, the only option was to define these variables twice.
One extern C and one constexpr C++ version.
To keep the code duplication as low as possible, the variables are first defined as compiler macro.
After that the variables are generated from the macros.
Lastly, the compiler macros are undefined, so that they do not leak and cannot be used anywhere else.

I am not happy about this, but it is the best option I could come up with.

All these boxes must be checked by the AUTHOR before requesting review:

• The PR is small enough to be reviewed easily. If not, consider splitting up the changes in multiple PRs.
• The title starts with one of the following prefixes: Documentation:, Bugfix:, Feature:, Improvement: or Other:.
• If the PR is related to an issue, make sure to link it.
• The author made sure that, as a reviewer, he/she would check all boxes below.

All these boxes must be checked by the REVIEWERS before merging the pull request:

As a reviewer please read through all the code lines and make sure that the code is fully understood, bug free, well-documented and well-structured.

General

• The reviewer executed the new code features at least once and checked the results manually.
• The code follows the t8code coding guidelines.
• New source/header files are properly added to the CMake files.
• The code is well documented. In particular, all function declarations, structs/classes and their members have a proper doxygen documentation.
• All new algorithms and data structures are sufficiently optimal in terms of memory and runtime (If this should be merged, but there is still potential for optimization, create a new issue).

Tests

• The code is covered in an existing or new test case using Google Test.
• The code coverage of the project (reported in the CI) should not decrease. If coverage is decreased, make sure that this is reasonable and acceptable.
• Valgrind doesn't find any bugs in the new code. This script can be used to check for errors; see also this wiki article.

If the Pull request introduces code that is not covered by the github action (for example coupling with a new library):

• Should this use case be added to the github action?
• If not, does the specific use case compile and all tests pass (check manually).

Scripts and Wiki

• If a new directory with source files is added, it must be covered by the script/find_all_source_files.scp to check the indentation of these files.
• If this PR introduces a new feature, it must be covered in an example or tutorial and a Wiki article.

License

• The author added a BSD statement to doc/ (or already has one).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.66%. Comparing base (ac41a20) to head (cfd5814).
⚠️ Report is 23 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1942      +/-   ##
==========================================
+ Coverage   75.64%   75.66%   +0.02%     
==========================================
  Files         103      105       +2     
  Lines       18623    18646      +23     
==========================================
+ Hits        14087    14109      +22     
- Misses       4536     4537       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[lenaploetzke]

Nice, I kind of like the solution.