QDoc: Preserve related links across module builds

Documentation for dependent modules relies on index files to link
to related non-members. Enabling WebXML output can change the
identities recorded in these files. A dependent module can then
associate a related non-member with the wrong declaration or fail
to resolve its links.

Give related non-members consistent identities in each index file,
regardless of the selected output formats. This preserves links to
their documentation when dependent modules load the index in a
separate run. WebXML output remains unchanged.

Task-number: QTBUG-150820
Fixes: QTBUG-151066
Pick-to: 6.12
Change-Id: Ied101cb33395dbe9d8d5d148eed0315716bb10e6
Reviewed-by: David Boddie <david.boddie@qt.io>