ty_utils: const expr build todo! on unsafe binder wrap/unwrap THIR

[SebTardif]

Bug

compiler/rustc_ty_utils/src/consts.rs recurse_build hits todo!("FIXME(unsafe_binders)") for PlaceUnwrapUnsafeBinder, ValueUnwrapUnsafeBinder, and WrapUnsafeBinder THIR nodes. Building a const expression through those ops ICEs.

Origin

Unsafe binder MIR/CTFE/codegen support:

rust-lang@fc1a9186dcd

Impact

ICE when lowering/building a const that passes through unsafe binder wrap/unwrap (feature-gated; part of #567 cluster).

Fix

Recurse into the source operand (wrap/unwrap is transparent for the constant value tree), matching THIR visitor treatment.