Make 'self' types concrete while typechecking arguments

[apiology]

Fixes a bunch of similar strong level typechecking errors on the Solargraph codebase.

Typical example:

module Solargraph2
  module Pin
    class Base
      # @param other [self]
      # @param attr [::Symbol]
      #
      # @sg-ignore
      # @return [undefined]
      def assert_same(other, attr); end

      # @param other [self]
      # @param attrs [Hash{Symbol => Object}]
      #
      # @return [self]
      def combine_with(other, attrs={})
      end
    end

    class BaseVariable < Base
      def combine_with(other, attrs={})
        attrs.merge({
          # Wrong argument type for Solargraph2::Pin::Base#assert_same: other expected Solargraph2::Pin::Base, received self
          mass_assignment: assert_same(other, :mass_assignment),
        })
      end
    end
  end
end

Depends on and includes:

• [regression] Fix resolution of a nested type case #1103