Initial impl of raw_assign_to_drop

[lukaslueg]

Fixes #4294

The lint simply checks all assignments via unsafe pointers where to dereferenced-type has drop-glue.

I'm somewhat unsure about what to call this thing - is it assign_raw_ptr_using_drop, raw_assign_drop, dropped_assign_raw, ... ?

Although some of the tests involve &mut as *mut, the lint does not make efforts to filter out situations where the raw pointer is derived from a known-safe source. The general assumption is that if we have a raw pointer at at all, and assign to the place behind the pointer, then all safety bets are off anyway (otherwise, one could have assigned via &mut to begin with).

changelog: [raw_assign_to_drop]: Initial impl

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