Fix the reporting of layout normalization failures in transmute checks

[meta-legend]

Fixes #149588.

"check_transmute" normalized the source and target types before computing their size skeletons. When that normalization failed, it created a {type error} placeholder and continued into the generic transmute-size diagnostic. This hid the actual layout failure and produced an unhelpful E0512 message.

This changes transmute checking to report the layout normalization failure directly and return early. The diagnostic now explains that rustc cannot determine the layout because the associated type cannot be normalized, instead of showing {type error} as the source type.

I also updated the existing ./tests/ui/layout/normalization-failure.rs test to cover the corrected diagnostic.

[rustbot]

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[rust-bors]

☔ The latest upstream changes (presumably #160238) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.