Allow Hidden fields to be indexed if thay are shown in "search_result" display

Please consider the related issue: #1977832

If a field is set as "Hidden" in the search_index display it would be ignored (and thus omitted) from SOLR index, this is OK for reducing the number of indexed fields; it is not correct if the omitted field is used for querying (or filtering) purposes and need to be indexed anyway not regarding its display setting in the search_index display.

Furthermore the "search_index" display is used to obtain the "content" SOLR Document field, thus if a field value is only needed for filtering the results (and it must not be added for full-text search) it will be indexed anyway.

The main idea behind the following patch is to index fields that are hidden in the search_index display but shown in the "search_result" display (the "search_result" display setting of a field is actually ignored at index time).

How to reproduce this issue:

• Create a content type Document
• Add a field named "myField" as a (integer) key|value pair with the list Module as "1|Drupal Rocks 2|Drupal Sks
• Set the field as "Hidden" in the search_index node display
• Build a searchpage, for the "custom filters" add "-im_field_myfield:2" (the aim is creating a search page excluding the "Drupal Sks" labeled nodes)
• Add few contents
• Try to search using hte previously creted page

No results will be found since the "field_myfield" won't never be indexed given that it is for the search_index display type