Fix UndocumentedDeclarationCheck linting issue in typecons.d

[Vladiwostok]

#9070 Addendum

This PR fixes some more linting errors found in the typecons.d file. Fixed a bug in dmd-as-a-lib D-Scanner that prevented it from detecting those issues in the first place.

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + phobos#9075"