Investigate whether we can remove more attributes in the linker #16678

We should create a list of all the attributes in our platform assembly (sorted by how often they appear), and go through each of them to answer if they're needed.

The investigation and implementation plan are documented in this analysis gist.

Except for attributes that are safe to remove unconditionally, removal must only happen when all these conditions are true:

• PrepareAssemblies=true
• PostProcessAssemblies=true
• The trimmable-static registrar is used

Implementation checklist:

• Foundation.ProtocolMemberAttribute — implemented in [linker] Trim [ProtocolMember] attributes with trimmable-static + PrepareAssemblies #26089; update its feature condition to include PostProcessAssemblies=true.
• Foundation.RequiredMemberAttribute — remove unconditionally.
  • [linker] Remove Foundation member attributes #26257
• Foundation.OptionalMemberAttribute — remove unconditionally.
  • [linker] Remove Foundation member attributes #26257
• System.Runtime.CompilerServices.ExtensionAttribute — remove under the shared eligibility gate.
• Foundation.ModelAttribute — remove under the shared eligibility gate.
• ObjCRuntime.NativeAttribute — remove under the shared eligibility gate.
• Foundation.RegisterAttribute — remove under the shared eligibility gate when IsDirectBinding is known to be true.
• Foundation.ExportAttribute — remove under the shared eligibility gate from eligible direct-binding NSObject wrappers and sealed BaseWrapper protocol wrappers, after verifying runtime block/delegate fallback paths.
  • [linker] Trim Export attributes after registration. #26313

Attributes removed by RemoveAttributesStep should be moved to trimmer xml files like ProtocolMemberAttribute, because the RemoveAttributesStep isn't executed for NativeAOT:

• Foundation.ProtocolAttribute
• ObjCRuntime.NativeNameAttribute
• ObjCRuntime.AdoptsAttribute