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/tests.rs

@@ -178,6 +178,7 @@ impl<T: Write> Write for Shared<T> {
     }
 }
 
+#[allow(rustc::untranslatable_diagnostic)] // no translation needed for tests
 fn test_harness(file_text: &str, span_labels: Vec<SpanLabel>, expected_output: &str) {
     create_default_session_globals_then(|| {
         let (handler, source_map, output) = create_test_handler();
@@ -192,7 +193,6 @@ fn test_harness(file_text: &str, span_labels: Vec<SpanLabel>, expected_output: &
             println!("text: {:?}", source_map.span_to_snippet(span));
         }
 
-        #[allow(rustc::untranslatable_diagnostic)]
         handler.span_err(msp, "foo");
 
         assert!(