diagnostics: missing CoroutineClosure in fulfillment span adjust and explain_borrow destructor

[SebTardif]

Bug

1. adjust_fulfillment_errors.rs only matches ty::Closure | ty::Coroutine when deciding whether a fulfillment error span overlaps the self_ty def; async closures (CoroutineClosure) miss this span adjustment.
2. explain_borrow.rs labels drop types as "closure" or "coroutine" but not "async closure" for CoroutineClosure.

Origin

Sibling CoroutineClosure sweep pattern (see #477, #478, #490).

Impact

Worse diagnostics for async closures (span attribution / wording), not ICE.

Fix

PR adding CoroutineClosure arms in both sites.