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_feature/src/builtin_attrs.rs

@@ -695,8 +695,8 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
     // Used by the `rustc::potential_query_instability` lint to warn methods which
     // might not be stable during incremental compilation.
     rustc_attr!(rustc_lint_query_instability, Normal, template!(Word), WarnFollowing, INTERNAL_UNSTABLE),
-    // Used by the `rustc::untranslatable_diagnostic` and `rustc::diagnostic_outside_of_impl` lints
-    // to assist in changes to diagnostic APIs.
+    // Used by the `rustc::diagnostic_outside_of_impl` lints to assist in changes to diagnostic
+    // APIs. Any function with this attribute will be checked by that lint.
     rustc_attr!(rustc_lint_diagnostics, Normal, template!(Word), WarnFollowing, INTERNAL_UNSTABLE),
     // Used by the `rustc::bad_opt_access` lint to identify `DebuggingOptions` and `CodegenOptions`
     // types (as well as any others in future).