Auto merge of #41349 - eddyb:ty-contents, r=nikomatsakis

Auto merge of #41349 - eddyb:ty-contents, r=nikomatsakis

rustc: replace TypeContents with two independent properties (is_freeze / needs_drop).

`InteriorUnsafe` / `interior_unsafe` was replaced with a private lang-item `Freeze` auto trait in libcore.

`OwnsDtor` / `needs_drop` was replaced with a specialized traversal that *doesn't* avoid caching results in case of a cycle, as the only cycles left can only occur in erroneous "types with infinite sizes", references and raw pointers not having destructors. Also, `Copy` is now checked at every step of the recursion.

r? @nikomatsakis