Overridden functions in Child Blueprints cannot be converted to/from pure functions

Overridden functions in Child Blueprints cannot be converted to/from pure functions.

Reproduced in 4.8.1 and Main (//depot/UE4/Promotable-CL-2601051)

1. Open any project
2. Create a new Actor Blueprint
3. Add a new function
4. Create a Child of the Blueprint
5. Override the function
6. In the parent, switch the function to "Pure" in the Details panel
7. Note the overridden function in the child

Result:
The overridden function is no longer affected by the change in the parent.

Expected:
The overridden function matches the original's purity.