Avoid using the libc++ internal `_LIBCPP_HAS_NO_INT128` in our tests. Reviewed By: #libc, ldionne Differential Revision: https://reviews.llvm.org/D117992
... from testing with MSVC's STL. Mostly truncation warnings and variables that are only used in `LIBCPP_ASSERT`. Differential Revision: https://reviews.llvm.org/D116878
Implement P1274R4 Reviewed By: Quuxplusone, Mordante, #libc Spies: jloser, lebedev.ri, mgorny, libcxx-commits, arichardson Differential Revision: https://reviews.llvm.org/D114074