allow repeated repr attributes

[ettolrach]

In principle, fixes #156029. That issue does mention the "inconsistency" of multiple "C" reprs mixed with a single integer repr being accepted, while multiple "Rust" reprs mixed with a single integer repr aren't. I do believe this is intentional, since the Rust repr cannot be mixed with integer reprs.

Also, I'm not sure whether the current behaviour regarding multiple align reprs should be changed. While it's inconsistent that multiple, mixed aligns are accepted (and the largest is used), multiple packed repr aren't allowed. I don't want to change this since it's making the compiler less permissive, not more, like the rest of my changes.

[rustbot]

Some changes occurred in compiler/rustc_passes/src/check_attr.rs

cc @jdonszelmann, @JonathanBrouwer

[rustbot]

r? @mejrs

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[qaijuang]

Thanks for the contribution! Would you mind adding tests? You can use the relevant repro examples from the issue as the basis for the tests.

See: https://rustc-dev-guide.rust-lang.org/tests/intro.html