Sourcekitten does not return extension declarations

[pcantrell]

For an extension of A, sourcekitten gives the original declaration of A instead of the extension declaration:

  {
    "key.kind" : "source.lang.swift.decl.extension",
    ...
    "key.annotated_decl" : "<Declaration>public final class List&lt;T : <Type usr=\"s:C10RealmSwift6Object\">Object<\/Type>&gt; : <Type usr=\"s:C10RealmSwift8ListBase\">ListBase<\/Type><\/Declaration>",
    ...

As far as I can tell, this prevents Jazzy from being able to show type constraints on extensions.

[jpsim]

Fixed in #441.