Analyze AxImporter for COM exposure

`AxImporter` is the class that generates the wrapper `AxHost` derived classes for ActiveX controls. In Core this is just a stub. COM references use this through the `AxImp` command line tool in addition to `TlbImp`.

In order for AOT/Trimming to work in these scenarios we'll need to write up exactly how these tools are working to understand the exposure to built-in COM support.

Presumably it is unlikely that we cannot make these work directly without changing the toolchain.