Auto merge of #115334 - RalfJung:transparent-aligned-zst, r=compiler-errors

repr(transparent): it's fine if the one non-1-ZST field is a ZST

This code currently gets rejected:
```rust
#[repr(transparent)]
struct MyType([u16; 0])
```
That clearly seems like a bug to me: `repr(transparent)` [got defined ](rust-lang/rust#77841 (comment)) as having any number of 1-ZST fields plus optionally one more field; `MyType` clearly satisfies that definition.

This PR changes the `repr(transparent)` logic to actually match that definition.

Author: bors