Inline direct calls to `mem::forget` in the frontend

[pcwalton]

When we see a direct call to mem::forget, we should just inline it in the frontend. This has two important benefits, one obvious and one not-so-obvious: (1) it reduces compilation time; (2) it can often avoid one memcpy, because of the way mem::forget is implemented.

[huonw]

Also addressed by #12634.

[luqmana]

Since we no longer translate intrinsics as function calls the only thing left here to just get rid of the wrapper in core::men and instead just make it a use core::intrinsics::forget.