Code quality

[trueqbit]

New Visual C++ 18 2026 as well as Clang 21 are out. In an attempt to compile sqlite_orm with these new toolchains I corrected and improved things along the way.

Fixes:

• Corrected deprecated spacing in user-defined literal operators, e.g. operator"" _cte() -> operator""_cte().

Generic improvements:

• Updated to clang-format 20.
• Used the single-argument version of static_assert, which was not possible with C++14 before.
• Removed a few unused type aliases in unit tests.
• Addressed a few warnings about unused variables.

Clang related:

• Temporarily disable spurious Clang warnings instead of suppressing them in code, which makes code cleaner and less likely to forget to suppress them on spot.
• Clang understands C++26 features even in C++17 mode, but it doesn't mean it understands other language features, so we need to be specific using them.
• Needed to disable compiling the R*Tree module with Clang because Clang chokes on using constexpr variables in lambdas. This is an unfixed issue reported several times to the Clang maintainers.

Clang MSVC:

• Made sqlite_orm compile with Clang MSVC. Specifically it still cannot use functions exported from DLLs as compile-time parameters, so we provide workarounds for it.