Rollup merge of #155294 - mejrs:coalescing, r=chenyukang

Add test for coalescing of diagnostic attribute duplicates

There is an existing [test](github.com/rust-lang/rust/blob/main/tests/ui/diagnostic_namespace/on_unimplemented/report_warning_on_duplicated_options.rs) that warnings for duplicates are emitted, but not for the messages themselves.