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_ast_lowering/src/lib.rs

@@ -2272,6 +2272,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         self.expr_block(block)
     }
 
+    #[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
     fn lower_array_length(&mut self, c: &AnonConst) -> hir::ArrayLen {
         match c.value.kind {
             ExprKind::Underscore => {