Fixes #15937 where YAML::Any will not correctly merge aliases into maps.

This file is parsed correctly:

hello: &single_alias world
single: *single_alias

map: &map_alias
  foo: bar
splatted: *map_alias
modified:
  <<: *map_alias
  food: bark
nonsense:
  <<: *single_alias

With this patch the result is:

hello: world
single: world
map:
  foo: bar
splatted:
  foo: bar
modified:
  foo: bar
  food: bark
nonsense:
  <<: world

With 1.16.3 the result is:

hello: world
single: world
map:
  foo: bar
splatted:
  foo: bar
modified:
  <<:
    foo: bar
  food: bark
nonsense:
  <<: world

(note the << submessage in modified that should have been splatted into the outer map).

I'm not sure if this is the best way to fix the issue, I don't really have a good understanding of how all the YAML parsing fits together, so I could totally be putting an inappropriate bandaid on this.