Fix static initialization order fiasco for global constants

[joka921]

QLever uses several global constants defined in the files Constants.h and ConstantsIndexBuilding.h. So far, many of them were defined as static const variables, which are prone to the so-called static initialization order fiasco. In particular, this caused unexpected test failures on macOS: This is now fixed by making all these constants constexpr, which makes the initialization order deterministic at compile time. To realize this, there is now a class ConstexprString and a function constexprStrCat for initializing constexpr std::string_view variables using compile-time string concatenation.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 89.45%. Comparing base (f986808) to head (fff6810). Report is 5 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1457 +/- ## ========================================== + Coverage 89.44% 89.45% +0.01% ========================================== Files 346 346 Lines 25262 25289 +27 Branches 3371 3373 +2 ========================================== + Hits 22595 22623 +28 + Misses 1507 1503 -4 - Partials 1160 1163 +3 ```

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[sonarcloud[bot]]

Quality Gate passed

Issues
3 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud