IWYU: Fix handling of <memory> standard header
An old workaround for `std::allocator_traits<>::value_type` lints from IWYU on `std::vector<>` usage breaks IWYU's handling of `<memory>`. Convert the workaround to use the same approach we already use for a workaround of `std::__decay_and_strip<>::::__type` lints. Then update the `<memory>` inclusions to follow the now-correct IWYU lints.
Showing with 104 additions and 69 deletions