Const drop

[fee1-dead]

The changes are pretty primitive at this point. But at least it works. ^-^

Problems with the current change that I can think of now:

• ~const Drop shouldn't change anything in the non-const world.
• types that do not have drop glues shouldn't fail to satisfy ~const Drop in const contexts. struct S { a: u8, b: u16 } This might not fail for needs_non_const_drop, but it will fail in rustc_trait_selection.
• The current change accepts types that have const Drop impls but have non-const Drop glue.

Fixes #88424.

Significant Changes:

• ~const Drop is no longer treated as a normal trait bound. In non-const contexts, this bound has no effect, but in const contexts, this restricts the input type and all of its transitive fields to either a) have a const Drop impl or b) can be trivially dropped (i.e. no drop glue)
• T: ~const Drop will not be linted like T: Drop.
• Instead of recursing and iterating through the type in rustc_mir::transform::check_consts, we use the trait system to special case ~const Drop. See rustc_trait_selection::...::candidate_assembly#assemble_const_drop_candidates and others.

Changes not related to const Dropping and/or changes that are insignificant:

• Node.constness_for_typeck no longer returns hir::Constness::Const for type aliases in traits. This was previously used to hack how we determine default bound constness for items. But because we now use an explicit opt-in, it is no longer needed.
• Removed is_const_impl_raw query. We have impl_constness, and the only existing use of that query uses HirId, which means we can just operate it with hir.
• ty::Destructor now has a field constness, which represents the constness of the destructor.

r? @oli-obk