Rollup merge of #139944 - shepmaster:eager-diagnostics, r=nnethercote

Move eager translation to a method on Diag

This will allow us to eagerly translate messages on a top-level diagnostic, such as a `LintDiagnostic`. As a bonus, we can remove the awkward closure passed into Subdiagnostic and make better use of `Into`.

r? `@nnethercote`

Author: matthiaskrgr