Rollup merge of #137808 - jswrenn:droppy-unsafe-fields, r=nnethercote

Do not require that unsafe fields lack drop glue

Instead, we adopt the position that introducing an `unsafe` field itself carries a safety invariant: that if you assign an invariant to that field weaker than what the field's destructor requires, you must ensure that field is in a droppable state in your destructor.

See:
- rust-lang/rfcs#3458 (comment)
- https://rust-lang.zulipchat.com/#narrow/channel/213817-t-lang/topic/unsafe.20fields.20RFC/near/502113897

Tracking Issue: #132922

Author: compiler-errors