[meta] Add TargetKind.constructor

[leoshusar]

Hi, I hope I haven't missed anything, but would it be possible to add TargetKind.constructor?

I believe there is currently no way to restrict annotation only for constructor.

[gmpassos]

Why is there no TargetKind.constructor defined?

@srawlins

[srawlins]

Someone forgot to add it, or decided it shouldn't be added.

[bwilkerson]

As with most things, we took a fairly agile approach and only added the target kinds that we needed for the annotations we had already defined at that point and were planning to use TargetKind to flag.

In hindsight that might have been a mistake because there are now uses of TargetKind that we hadn't anticipated that make adding new kinds a breaking change. What we need to do at some point is to extend TargetKind to have all of the kinds that make sense so that we minimize the number of breaking changes (as opposed to adding new kinds individually). Unfortunately, I don't have a timeframe for when such a task might happen.