Incomplete Type Index / Bug 109084

_(Copy of https://github.com/vi-eclipse/Eclipse-JDT/issues/4)_
### Current Behavior

In some cases (with unknown reasons), the type index does not contain all available types, so the type search does not find some types that actually exist.
Rebuilding the index via the preferences does not resolve the issue.


The bug has been existing for a long time and does not seem to be fixed by now: https://bugs.eclipse.org/bugs/show_bug.cgi?id=109084

### Expected Behavior

In the best case, the type index should properly contain the expected types.
At least, the "rebuild index" operation in the preferences should properly rebuild the index.

### Existing Workaround
Delete \.metadata\.plugins\org.eclipse.jdt.core\*.index, and restart Eclipse.