We should update our ILLinkTrim files given dotnet/linker#1257.

1. Rename the files in source control from ILLinkTrim.xml to ILLink.Descriptors.xml
   • For the _LibraryBuild files, we should name them ILLink.Descriptors.LibraryBuild.xml or similar
2. When embedding the file into the assembly, we should rename the resource from $(AssemblyName).xml to ILLink.Descriptors.xml.
   • See

     runtime/eng/illink.targets

     Lines 48 to 52 in 0b416bc

     	<ItemGroup Condition="'$(ILLinkTrimXml)' != ''">
     	<EmbeddedResource Include="$(ILLinkTrimXml)">
     	<LogicalName>$(AssemblyName).xml</LogicalName>
     	</EmbeddedResource>
     	</ItemGroup>