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/expand.rs

@@ -784,6 +784,7 @@ impl<'a, 'b> MacroExpander<'a, 'b> {
         })
     }
 
+    #[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
     fn gate_proc_macro_attr_item(&self, span: Span, item: &Annotatable) {
         let kind = match item {
             Annotatable::Item(_)
@@ -826,6 +827,7 @@ impl<'a, 'b> MacroExpander<'a, 'b> {
         }
 
         impl<'ast, 'a> Visitor<'ast> for GateProcMacroInput<'a> {
+            #[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
             fn visit_item(&mut self, item: &'ast ast::Item) {
                 match &item.kind {
                     ItemKind::Mod(_, mod_kind)
@@ -1690,6 +1692,7 @@ impl<'a, 'b> InvocationCollector<'a, 'b> {
 
     // Detect use of feature-gated or invalid attributes on macro invocations
     // since they will not be detected after macro expansion.
+    #[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
     fn check_attributes(&self, attrs: &[ast::Attribute], call: &ast::MacCall) {
         let features = self.cx.ecfg.features;
         let mut attrs = attrs.iter().peekable();