New lint: suggest `ptr::read` instead of `mem::replace(..., uninitialized())`
resolves: #5575
changelog: improvements to `MEM_REPLACE_WITH_UNINIT`:
- add a new test case in `tests/ui/repl_uninit.rs` to cover the case of replacing with `mem::MaybeUninit::uninit().assume_init()`.
- modify the existing `MEM_REPLACE_WITH_UNINIT` when replacing with `mem::uninitialized` to suggest using `ptr::read` instead.
- lint with `MEM_REPLACE_WITH_UNINIT` when replacing with `mem::MaybeUninit::uninit().assume_init()`