PR: auto-generated building element proxy type

### Problem Description

There seems to be a case where Revit auto generates a bunch of IfcBuildingElementProxy elements, each with its own IfcBuildingElementProxyType. I believe in this case it is some sort of curtain wall reveal or other profile being used as part of a curtain wall. This ends up with a model with hundreds of IfcBuildingElementProxyTypes. They cannot be edited in Revit (i.e. cannot set IfcExportAs) so we cannot set any properties.

This means that clients will see a bunch of generic proxy elements missing all the data that they request.


### Revit Version

2023.1.x

### IFC for Revit Addon Version

23.2.5

### Windows Version

11 22H2

Start by reproducing the reported curtain-wall case in Revit 2023.1.x with IFC for Revit 23.2.5, then trace how auto-generated IfcBuildingElementProxy elements and their types are exported. Done means the generated proxies no longer leave clients with generic elements missing the requested data; the issue names no source files or tests to run.

Written by the indexing model from the issue text.