Implement pinned drop sugar

[P8L1]

Implements fn drop(&pin mut self) as sugar for Drop::pin_drop.

The resolver recognizes the sugar only for the actual #[lang = "drop"] trait, maps accepted impl items to effective pin_drop resolution, and passes marked impl item IDs to lowering. Lowering then emits HIR with ident pin_drop, so existing type checking, Drop validation, drop glue, and direct-call checks remain unchanged.

Drop identity is handled without a hardcoded core::ops::drop::Drop path check. Local #[lang = "drop"] traits are recorded from raw AST attributes, and external Drop identity is checked through external lang-item metadata via defined_lang_items. This avoids calling the all-crates tcx.lang_items() query from resolver.

This intentionally preserves the base pinned-drop behavior where #[pin_v2] types must use pin_drop, while non-#[pin_v2] types may still implement pin_drop.

[P8L1]

@frank-king have you had time to review this by any chance?

[frank-king]

I'm not the proper reviewer for this PR, could you please open a PR directly merging to the upstream rust-lang/rust repo?

(Note, you can open a draft PR and mark it ready since rust-lang#144537 is merged)