Respect embedded xml files from copy assemblies

[sbomer]

The embedded xml files should be respected from copy assemblies the same way they are for link assemblies. I'm actually not sure why we respect them for AddBypassNGenUsed - I would expect that for assemblies that do not end up being used (even if they have action CopyUsed or AddBypassNGenUsed), we don't add roots from xml.

Necessary to make dotnet/wpf#1387 work. We're adding roots to WPF assemblies to work around the linker's inability to process C++/CLI assemblies (#658).

@marek-safar PTAL.

[sbomer]

Tell mode form:

Description

This fix will let the linker read embedded xml root descriptors that come from "copy" assemblies - which is everything outside the core framework in the .NET Core use case. This allows WPF to ship assemblies with roots to keep dependencies of unanalyzed C++/CLI assemblies.

Customer Impact

Together with dotnet/wpf#1387, System.Diagnostics.Debug, netstandard, and mscorlib will be kept in every WPF app. This will make more WPF apps that use DirectWriteForwarder.dll or System.Printing.dll work with PublishTrimmed=true.

Regression?

Not a regression.

Risk

Low risk. Potentially some WPF users will be unhappy that these dependencies are being kept even if they aren't strictly necessary for their apps. However I expect that most apps will need these - we've had a number of reports where very simple apps (with single controls) required System.Diagnostics.Debug, and netstandard and mscorlib shouldn't cause much of a problem because they are fairly small.

[vatsan-madhavan]

System.Diagnostics.Debug is touched in a module-constructor in DirectWriteForwarder; DirectWriteForwarder is used in the rendering of any (every) text - this dependency is certain to be touched in even a simple application.

[vatsan-madhavan]

@sbomer, @marek-safar, We'd love to be able to test a core-sdk build that contains fixed bits before snapping Preview 8 from master -> release/3.0 on Monday. Can we get this fix in core-sdk sometime today?

[sbomer]

@marek-safar I think this is good to go once ci is green (the new test passes on my machine). PTAL.