Rewrite the untranslatable_diagnostic lint.

Currently it only checks calls to functions marked with
`#[rustc_lint_diagnostics]`. This commit changes it to check calls to
any function with an `impl Into<{D,Subd}iagMessage>` parameter. This
greatly improves its coverage and doesn't rely on people remembering to
add `#[rustc_lint_diagnostics]`.

The commit also adds `#[allow(rustc::untranslatable_diagnostic)`]
attributes to places that need it that are caught by the improved lint.
These places that might be easy to convert to translatable diagnostics.

Finally, it also:
- Expands and corrects some comments.
- Does some minor formatting improvements.
- Adds missing `DecorateLint` cases to
  `tests/ui-fulldeps/internal-lints/diagnostics.rs`.

compiler/rustc_expand/src/proc_macro_server.rs

@@ -517,6 +517,9 @@ impl server::FreeFunctions for Rustc<'_, '_> {
             Diag::new(&self.psess().dcx, diagnostic.level.to_internal(), message);
         diag.span(MultiSpan::from_spans(diagnostic.spans));
         for child in diagnostic.children {
+            // This message comes from another diagnostic, and we are just reconstructing the
+            // diagnostic, so there's no need for translation.
+            #[allow(rustc::untranslatable_diagnostic)]
             diag.sub(child.level.to_internal(), child.message, MultiSpan::from_spans(child.spans));
         }
         diag.emit();