fix: add CoroutineClosure and Error arms to simplify_nounwind_call

[SebTardif]

Summary

Fixes #477

simplify_nounwind_call in instsimplify.rs matches on body_ty.kind() to determine the ABI of a callee, but the match is missing ty::CoroutineClosure(..) => ExternAbi::RustCall and ty::Error(_) => return. The catch-all _ => bug!(...) causes an ICE when the callee is an async closure (which desugars to CoroutineClosure).

Failure scenario

When MIR optimization encounters a Call terminator whose callee is a coroutine closure body, type_of(def_id) returns ty::CoroutineClosure. The match falls through to bug!("unexpected body ty"), causing an Internal Compiler Error. Async closures have been stable since Rust 1.85, making this reachable on stable.

Fix

Add the two missing arms, matching the pattern used by three sibling functions in the same crate (validate.rs:57, abort_unwinding_calls.rs:55, ffi_unwind_calls.rs:30) that all include both CoroutineClosure and Error handling.

Bug origin

The CoroutineClosure variant was added to TyKind as part of the async closures feature. The three sibling functions were updated, but simplify_nounwind_call was missed.