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.