Port `#[rustc_pub_transparent]` to the new attribute system
Very similar to rust-lang/rust#142498.
This is a part of rust-lang/rust#131229, so
r? ````@jdonszelmann````
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For reference, the `#[rustc_pub_transparent]` attribute was created by me back in rust-lang/rust#129487.
As mentioned back in https://github.com/rust-lang/rust/pull/129487#discussion_r1730120385, this attribute does not check that it is applied to an ADT, because it checks that `#[repr(transparent)]` is also applied to the same item, which, in turn, should check for ADT.
Jacob Pratt
431a822938