Annotate framework assemblies as trimmable (#48428)

* Annotate framework assemblies as trimmable

This will opt them into trimming with the behavior described at https://github.com/mono/linker/blob/main/docs/design/trimmed-assemblies.md#assemblymetadataistrimmable-true.

* Add a condition to let projects opt out

A project just needs to set <SetIsTrimmable>false</SetIsTrimmable>, and
it won't get the trimmable metadata.

I also got rid of the SkipFrameworkAssemblyMetadata check since I don't
think the IsTrimmable metadata should be tied to whether we are building
a framework assembly, and this property was unused as far as I could tell.

eng/versioning.targets

@@ -20,6 +20,9 @@
     <AssemblyMetadata Include="PreferInbox">
       <Value>True</Value>
     </AssemblyMetadata>
+    <AssemblyMetadata Include="IsTrimmable" Condition="'$(SetIsTrimmable)' != 'false'">
+      <Value>True</Value>
+    </AssemblyMetadata>
   </ItemGroup>
 
   <!-- Adds SupportedOSPlatform attribute for Windows Specific libraries -->
@@ -83,7 +86,7 @@
       </AssemblyAttribute>
 
       <AssemblyAttribute Include="System.Reflection.AssemblyMetadata"
-                         Condition="'$(SkipFrameworkAssemblyMetadata)' != 'true' and '@(AssemblyMetadata)' != ''">
+                         Condition="'@(AssemblyMetadata)' != ''">
         <_Parameter1>%(AssemblyMetadata.Identity)</_Parameter1>
         <_Parameter2>%(AssemblyMetadata.Value)</_Parameter2>
       </AssemblyAttribute>

src/libraries/System.Private.CoreLib/src/Internal/AssemblyAttributes.cs

@@ -22,5 +22,6 @@
 
 [assembly: AssemblyMetadata("Serviceable", "True")]
 [assembly: AssemblyMetadata(".NETFrameworkAssembly", "")]
+[assembly: AssemblyMetadata("IsTrimmable", "True")]
 
 [assembly: NeutralResourcesLanguage("en-US")]