Previously, the following situation would throw an error:
* A search contains a `collapse` on a particular field.
* The search spans multiple indices, and in one index the field is mapped as a
  concrete field, but in another it is a field alias.

The error occurs when we attempt to merge `CollapseTopFieldDocs` across shards.
When merging, we validate that the name of the collapse field is the same across
shards. But the name has already been resolved to the concrete field name, so it
will be different on shards where the field was mapped as an alias vs. shards
where it was a concrete field.

This PR updates the collapse field name in `CollapseTopFieldDocs` to the
original requested field, so that it will always be consistent across shards.

Note that in elastic#32648, we already made a fix around collapsing on field aliases.
However, we didn't test this specific scenario where the field was mapped as an
alias in only one of the indices being searched.