Rollup merge of #121382 - nnethercote:rework-untranslatable_diagnostic-lint, r=davidtwco

Rework `untranslatable_diagnostic` lint

Currently it only checks calls to functions marked with `#[rustc_lint_diagnostics]`. This PR changes it to check calls to any function with an `impl Into<{D,Subd}iagnosticMessage>` parameter. This greatly improves its coverage and doesn't rely on people remembering to add `#[rustc_lint_diagnostics]`. It also lets us add `#[rustc_lint_diagnostics]` to a number of functions that don't have an `impl Into<{D,Subd}iagnosticMessage>`, such as `Diag::span`.

r? ``@davidtwco``

src/parse/session.rs

@@ -360,6 +360,7 @@ mod tests {
         }
 
         fn build_diagnostic(level: DiagnosticLevel, span: Option<MultiSpan>) -> DiagInner {
+            #[allow(rustc::untranslatable_diagnostic)] // no translation needed for empty string
             let mut diag = DiagInner::new(level, "");
             diag.messages.clear();
             if let Some(span) = span {