Rollup merge of #113091 - GuillaumeGomez:prevent-cfg-merge-reexport, r=rustdoc

Don't merge cfg and doc(cfg) attributes for re-exports

Fixes #112881.

## Explanations

When re-exporting things with different `cfg`s there are two things that can happen:

 * The re-export uses a subset of `cfg`s, this subset is sufficient so that the item will appear exactly with the subset
 * The re-export uses a non-subset of `cfg`s (e.g. like the example I posted just above where the re-export is ungated), if the non-subset `cfg`s are active (e.g. compiling that example on windows) then this will be a compile error as the item doesn't exist to re-export, if the subset `cfg`s are active it behaves like 1.

### Glob re-exports?

**This only applies to non-glob inlined re-exports.** For glob re-exports the item may or may not exist to be re-exported (potentially the `cfg`s on the path up until the glob can be removed, and only `cfg`s on the globbed item itself matter), for non-inlined re-exports see https://github.com/rust-lang/rust/issues/85043.

cc `@Nemo157`
r? `@notriddle`

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