add incremental test for issue 135514
r? `@compiler-errors` as requested in https://github.com/rust-lang/rust/issues/135514#issuecomment-2591614811
This adds parts of `@steffahn's` repro as an incremental test for #135514. I had initially added the actual exploitation of the issue into the safe transmute, but removed it because it's not exactly needed for such a test. I can add it back if you'd like.
I've verified that the test fails with https://github.com/rust-lang/rust/pull/133828 reverted.
5bebace822