simplify_nounwind_call missing CoroutineClosure arm causes ICE on async closures

[SebTardif]

Bug

simplify_nounwind_call in compiler/rustc_mir_transform/src/instsimplify.rs matches on body_ty.kind() to determine the ABI of a callee, but is missing two arms:

• ty::CoroutineClosure(..) => ExternAbi::RustCall
• ty::Error(_) => return

The catch-all _ => bug!("unexpected body ty: {body_ty:?}") causes an ICE when the callee is an async closure body (which desugars to CoroutineClosure), or when the type is an error type from a previous compilation error.

Evidence

Three structurally identical functions in the same crate perform the same type_of(def_id).skip_binder() -> body_ty.kind() dispatch and ALL include the missing arms:

1. validate.rs (lines 57-62): has ty::CoroutineClosure(..) => ExternAbi::RustCall and ty::Error(_) => return
2. abort_unwinding_calls.rs (lines 55-59): has both arms
3. ffi_unwind_calls.rs (lines 30-34): has both arms (Error returns false)

Only instsimplify.rs was missed when CoroutineClosure was added to TyKind.

Impact

ICE (Internal Compiler Error) when async closures are used in certain call patterns that reach the simplify_nounwind_call MIR optimization pass. Async closures have been stable since Rust 1.85. The missing ty::Error arm also causes ICE instead of graceful degradation when compilation already has errors.

PR #481