Deinitializers are not shown in document symbols

[ahoppen]

Eg. if you have

class Foo {
  deinit {}
}

Then deinit should be shown in the document symbols, but it is not. This would need to be added to the DocumentSymbolsFinder.

[ahoppen]

Synced to Apple’s issue tracker as rdar://128715297

[ahoppen]

Fixed by #1397