Trimmer doesn't keep `TypeMapAssemblyTargetAttribute`s

[jtschuster]

The trimmer will track and mark the TypeMap and TypeMapAssociation attributes, but it doesn't seem to mark the TypeMapAssemblyTarget attributes anywhere.

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/illink
See info in area-owners.md if you want to be subscribed.