[libcxx][test] Silence nodiscard warnings (#154622)
MSVC's STL marks `std::make_shared`, `std::allocate_shared`,
`std::bitset::to_ulong`, and `std::bitset::to_ullong` as
`[[nodiscard]]`, which causes these libcxx tests to emit righteous
warnings. They should use the traditional `(void)` cast technique to
ignore the return values.
