rustc_public: MIR WrapUnsafeBinder/UnwrapUnsafeBinder todo! in SMIR convert

[SebTardif]

Bug

compiler/rustc_public/src/unstable/convert/stable/mir.rs hits todo!("FIXME(unsafe_binders):") for:

• Rvalue::WrapUnsafeBinder
• ProjectionElem::UnwrapUnsafeBinder

rustc_public MIR enums did not yet expose these variants, so stable conversion could not complete.

Origin

Unsafe binder MIR/CTFE/codegen:

rust-lang@fc1a9186dcd

Impact

ICE when SMIR/rustc_public lowers MIR that uses unsafe binder wrap/unwrap (feature-gated; part of #567).

Fix

Add Rvalue::WrapUnsafeBinder and ProjectionElem::UnwrapUnsafeBinder to rustc_public, with stable convert, internal convert, pretty-print, and visitor support.