Address "never type fallback" lint in transmute macros

[joshlf]

In #1338, we pinned to a new compiler version that emits a new lint. That PR just updates our UI test files, and we accepted the PR since it is not strictly a soundness issue. However, it may indicate that our macros are fragile. This issue tracks digging in deeper and seeing whether we can restructure our macros to never emit that lint.

Breadcrumbs:

• Tracking Issue for Rust 2024: Make ! fall back to ! rust-lang/rust#123748 (comment)
• Make macros robust against never type fallback #1339