fix: add CoroutineClosure to describe_field_from_ty for capture names

[SebTardif]

Summary

Fixes #478

describe_field_from_ty in compiler/rustc_borrowck/src/diagnostics/mod.rs matches ty::Closure | ty::Coroutine to resolve captured variable names in borrow checker diagnostics, but misses ty::CoroutineClosure. This causes borrow checker errors involving async closure captures to show "field 0" instead of the actual variable name.

Failure scenario

When the borrow checker reports an error involving a captured variable inside an async closure, the diagnostic shows the numeric field index (e.g., "cannot borrow field 0 as mutable") instead of the variable name (e.g., "cannot borrow data as mutable"). This makes errors harder to understand.

Fix

Add ty::CoroutineClosure(def_id, _) to the existing match arm alongside ty::Closure and ty::Coroutine. The closure_captures API works identically for all three closure types.

Bug origin

The CoroutineClosure variant was added for async closures (stable since Rust 1.85). This diagnostic path was not updated.