Add option to keep/remove COM interfaces

[sbomer]

Adds an illink option that can be used to remove COM interfaces (the existing behavior is to preserve COM interfaces on marked types). If a type implements a COM interface that is also marked elsewhere, it will still be kept even with --keep-com-interfaces false. This includes windows runtime interfaces under the same option name.

This eliminates some trim warnings in winforms due to trim-incompatible code in Control's implementations of COM interfaces like IPersistPropertyBag. Experimentally, this still produces a working app for the scenario we've been testing. For now, this just introduces the option without setting it by default anywhere (or exposing it in MSBuild) so that we can opt in from the winforms scenario. We might consider setting it wherever BuiltInComInteropSupport is false.

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[jkoritzinsky]

I have another idea to avoid silent bad RCWs/CCWs: Given that this would break when built-in COM is enabled, could we remove the attributes/metadata that make an interface a COM interface if any of its members are trimmed? Then we wouldn't have the bad scenario where someone could say "don't keep COM interfaces, but keep built-in COM enabled", as if a COM interface is trimmed, it's no longer a COM interface.

[sbomer]

Thanks for the suggestion, I opened #101128 to track that, but I don't think we need it for this change. The problem you mentioned exists with the current behavior - even when keeping COM interfaces, we may remove interface members. I think we are covered by the fact that built-in COM is not supported when trimming.