XmlSerializer doesn't preserve constructors and properties on the serialized Type graph

[eerhardt]

When an application is PublishTrimmed=true and TrimMode=link, and XmlSerializer is deserializing/serializing Types that are in assemblies being trimmed, properties of the serialized Types are being trimmed by the ILLinker.

In order to support XmlSerializer in a fully trimmed app, we would either need to:

1. Get the ILLinker to support preserving properties recursively on Types
   • [feature/dataflow] Support DynamicallyAccessedMemberKinds.RecursiveProperties linker#1087
2. Create a Source Generator for XmlSerializer
   • see User Story: Source generators are added to common libraries to improve performance #43545

Note: this probably isn't required for 6.0, because Xamarin and Blazor WASM SDKs only trim the "core libraries", and leave any user code untrimmed by default.

[eerhardt]

We marked the XmlSerializer APIs as RequiresUnreferencedCode, which will give the users an ILLink warning. Closing.