Auto merge of #96853 - davidtwco:diagnostic-translation-unit-and-more-porting, r=oli-obk

diagnostics: port more diagnostics to derive + support for `()` fields

- Extend diagnostic derive so that spanless subdiagnostics (e.g. some uses of `help`/`note`) can be applied via attributes to fields of type `()` (currently spanless subdiagnostics are applied via attributes on the diagnostic struct itself). A consequence of this is that `Option<()>` fields can be used to represent optional spanless subdiagnostics, which are sometimes useful (e.g. for a `help` that should only show on nightly builds).
- Simplify the "explicit generic args with impl trait" diagnostic struct (from #96760) using support for `Option<()>` spanless subdiagnostics.
- Change `DiagnosticBuilder::set_arg`, used to provide context for Fluent messages, so that it takes anything that implements `IntoDiagnosticArg`, rather than `DiagnosticArgValue` - this improves the ergonomics of manual implementations of `SessionDiagnostic` which are translatable.
- Port "the type parameter `T` must be explicitly specified", "manual implementations of `X` are experimental", "could not resolve substs on overridden impl" diagnostics to diagnostic structs.
- When testing macros from `rustc_macros` in `ui-fulldeps` tests, sometimes paths from the compiler source tree can be shown in error messages - these need to be normalized in `compiletest`.

r? `@oli-obk`
cc `@pvdrz`

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