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_passes/src/check_attr.rs

@@ -1074,6 +1074,7 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
     /// of one item. Read the documentation of [`check_doc_inline`] for more information.
     ///
     /// [`check_doc_inline`]: Self::check_doc_inline
+    #[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
     fn check_doc_attrs(
         &self,
         attr: &Attribute,
@@ -1756,6 +1757,7 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
     }
 
     /// Checks if the `#[repr]` attributes on `item` are valid.
+    #[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
     fn check_repr(
         &self,
         attrs: &[Attribute],

compiler/rustc_passes/src/check_const.rs

@@ -77,6 +77,7 @@ impl<'tcx> CheckConstVisitor<'tcx> {
     }
 
     /// Emits an error when an unsupported expression is found in a const context.
+    #[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
     fn const_check_violated(&self, expr: NonConstExpr, span: Span) {
         let Self { tcx, def_id, const_kind } = *self;
 

compiler/rustc_passes/src/entry.rs

@@ -114,6 +114,7 @@ fn find_item(id: ItemId, ctxt: &mut EntryContext<'_>) {
     }
 }
 
+#[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
 fn configure_main(tcx: TyCtxt<'_>, visitor: &EntryContext<'_>) -> Option<(DefId, EntryFnType)> {
     if let Some((def_id, _)) = visitor.start_fn {
         Some((def_id.to_def_id(), EntryFnType::Start))