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_lint/src/let_underscore.rs

@@ -104,6 +104,7 @@ const SYNC_GUARD_SYMBOLS: [Symbol; 3] = [
 ];
 
 impl<'tcx> LateLintPass<'tcx> for LetUnderscore {
+    #[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
     fn check_local(&mut self, cx: &LateContext<'_>, local: &hir::Local<'_>) {
         if matches!(local.source, rustc_hir::LocalSource::AsyncFn) {
             return;