Rollup merge of #116882 - fmease:rustdoc-generalized-priv-repr-heuristic, r=rustdoc

rustdoc: hide `#[repr]` if it isn't part of the public ABI

> [!IMPORTANT]
> Temporarily stacked on top of PR rust-lang/rust#146527; only the last commit is relevant!

Follow-up to rust-lang/rust#115439.
Unblocks rust-lang/rust#116743, CC ``@dtolnay.``

Fixes rust-lang/rust#66401.
Fixes rust-lang/rust#128364.
Fixes rust-lang/rust#137440.

Only display the representation `#[repr(REPR)]` (where `REPR` is not `Rust` or `transparent`) of a given type if none of its variants (incl. the synthetic variants of structs) are `#[doc(hidden)]` and all of its fields are public and not `#[doc(hidden)]` since it's likely not meant to be considered part of the public ABI otherwise.

`--document-{private,hidden}-items` works as expected in this context, too.

Moreover, we now also factor in the presence of `#[doc(hidden)]` when checking whether to show `repr(transparent)` or not.

Author: matthiaskrgr