Show whether a type has drop glue (or a direct drop impl) on hover

[Veykril]

Drop glue is one of those implicit things thats hard to verify manually, rust-analyzer should allow you to easily tell whether there is drop glue or not

[ChayimFriedman2]

Note that we need to know about drop existence for borrowck anyway (eventually), so adding it to the IDE is trivial enough.

[ChayimFriedman2]

Actually, there is a conflict between IDE and compiler needs: when there are type parameters, we want IDE to not always show "has drop glue", but compiler needs to mark them as having drop.