This change adds a new preference to org.eclipse.jdt.core:

disableRestrictedFileIndexing

If the preference is set to 'true' and a file is restricted,
index requests for the file will be ignored.

Example preference for product customization:

org.eclipse.jdt.core/disableRestrictedFileIndexing=true

Fixes: https://github.com/eclipse-jdt/eclipse.jdt.core/issues/4970