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/context.rs

@@ -621,12 +621,13 @@ pub trait LintContext {
     /// Note that this function should only be called for [`LintExpectationId`]s
     /// retrieved from the current lint pass. Buffered or manually created ids can
     /// cause ICEs.
-    #[rustc_lint_diagnostics]
     fn fulfill_expectation(&self, expectation: LintExpectationId) {
         // We need to make sure that submitted expectation ids are correctly fulfilled suppressed
         // and stored between compilation sessions. To not manually do these steps, we simply create
-        // a dummy diagnostic and emit is as usual, which will be suppressed and stored like a normal
-        // expected lint diagnostic.
+        // a dummy diagnostic and emit it as usual, which will be suppressed and stored like a
+        // normal expected lint diagnostic.
+        #[allow(rustc::diagnostic_outside_of_impl)]
+        #[allow(rustc::untranslatable_diagnostic)]
         self.sess()
             .dcx()
             .struct_expect(