fix: add CoroutineClosure to maybe_drop_guard type list

[SebTardif]

Summary

Fixes #479

maybe_drop_guard in liveness.rs determines whether a type should suppress false unused variable lint warnings when used as a drop guard. It explicitly lists ty::Closure and ty::Coroutine but misses ty::CoroutineClosure, causing false positive warnings when async closures are used as drop guards.

Failure scenario

When a user writes let _guard = create_async_closure(); as a drop guard pattern, the lint pass does not recognize the async closure type as a valid drop guard and emits a spurious unused variable warning.

Fix

Add ty::CoroutineClosure(..) to the matches! list, alongside ty::Closure and ty::Coroutine.

Bug origin

The CoroutineClosure variant was added to TyKind for async closures (stable since Rust 1.85). The maybe_drop_guard function was not updated to include the new variant.