This helper method works for all types, falling back to a query for
`TyKind::Adt`s to determine whether the implement the
`{Partial,}StructuralEq` traits.

Co-Authored-By: Nvarkor <github@varkor.com>

* Handle arrays with const-generic lengths
* Use closure for repeated code.

* Handle cycles in `needs_drop` correctly
* Normalize types when computing `needs_drop`
* Move queries from rustc to rustc_ty

For now, this is all the crate contains, but more
attribute logic & types will be moved there over time.

This feature adds `X..`, `..X`, and `..=X` patterns.

2. mir::Mutability -> ast::Mutability.