Check for live drops in constants after drop elaboration
Resolves#66753.
This PR splits the MIR "optimization" pass series in two and introduces a query–`mir_drops_elaborated_and_const_checked`–that holds the result of the `post_borrowck_cleanup` analyses and checks for live drops. This query is invoked in `rustc_interface` for all items requiring const-checking, which means we now do `post_borrowck_cleanup` for items even if they are unused in the crate.
As a result, we are now more precise about when drops are live. This is because drop elaboration can e.g. eliminate drops of a local when all its fields are moved from. This does not mean we are doing value-based analysis on move paths, however; Storing a `Some(CustomDropImpl)` into a field of a local will still set the qualifs for that entire local.
r? @oli-obk
5c61a8dc34