Rollup merge of #112662 - Vanille-N:symbol_unique, r=RalfJung

`#[lang_item]` for `core::ptr::Unique`

Tree Borrows is about to introduce experimental special handling of `core::ptr::Unique` in Miri to give it a semantics.
As of now there does not seem to be a clean way (i.e. other than `&format!("{adt:?}") == "std::ptr::Unique"`) to check if an `AdtDef` represents a `Unique`.

r? `@RalfJung`

Draft: making a lang item

Author: compiler-errors