Rollup merge of #122315 - nnethercote:multiple-into-diag-message, r=Nilstrieb

Allow multiple `impl Into<{D,Subd}iagMessage>` parameters in a function.

The internal diagnostic lint currently only allows one, because that was all that occurred in practice. But rust-lang/rust-clippy/pull/12453 wants to introduce functions with more than one, and this limitation is getting in the way.

r? `@Nilstrieb`