[libc++][test] Fix simple warnings (#74186)

Found while running libc++'s tests with MSVC's STL. This fixes 3 kinds of warnings:

- Add void-casts to fix `-Wunused-variable` warnings.
- Avoid sign/truncation warnings in `ConvertibleToIntegral.h`.
- Add `TEST_STD_AT_LEAST_23_OR_RUNTIME_EVALUATED` to avoid mixing preprocessor 
  and runtime tests.
- Cleanup: Add `TEST_STD_AT_LEAST_20_OR_RUNTIME_EVALUATED` for
  consistency.