Expose method reference receivers

[vinistock]

This PR exposes the resolved declaration for method receivers. When there's no receiver or it's unresolved, we return nil. This helps us provide more accurate find references in the Ruby LSP.

Note: exposing the receiver as Declaration? is probably not 100% correct, but since we cannot know the type of method receivers until there's type inference, it's a decent mid-point.

I also prototyped changing the receiver to be a constant reference ID (instead of a name ID), but it leads to a lot of other issues. For example:

class Foo
  def bar
    # In this method reference, the receiver is Foo. However, is the `self` considered
    # a constant reference to Foo? Not really...
    #
    # It would be pretty surprising to have this should up in find references and
    # much worse in a rename request.
    self.baz
  end
end

Maybe, in the future, the self would be something like a TypeRef? Anyway, it's clearly too early to model this so I went with a compromise.

[vinistock]

• Expose method reference receivers #754 👈 (View in Graphite)
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