v6-08: fBits read: preserve kIsOnHeap, always set kNotDeleted.

[pcanal]

Rather than reading from the file the value of kIsOnHeap, preserve the value that was calculated at object creation time (i.e. in the current execution). For example, for an embedded object (inside an object created on the heap or stack), the bit always need to be off (i.e. it can never be explicitly deleted)

Fix #15964 ?

This Pull request:

Changes or fixes:

Checklist:

• [ ] tested changes locally
• [ ] updated the docs (if necessary)

This PR fixes #