Allow .d.ts files to represent anonymous class expressions with `private` members

[trusktr]

Re-opening of #30355

That other issue is labeled "working as intended" so instead of this feature being a bug report, it is a feature request.

[RyanCavanaugh]

Wouldn't it be cool if this had some text in it?

[trusktr]

It doesn't need, the other issue has details, but it is closed. I just think that this is a valid problem that is faced much too often by TS devs.

[web-padawan]

Related to #17293 (actually the same) - see #17293 (comment)

Related to #17744 - see my comment explaining why this is important #17744 (comment)

[RonnieLincoln]

any process?

Encountered this issue when trying Builder Pattern.