The registrar requires the availability attributes to work properly, which is
non-trivial when the linker is being used, because the linker runs before the
registrar, and will remove availability attributes.
For this reason we store the availability attributes separately when the
linker removes them so that the registrar can still find them, but
unfortunately it's not enough to store the CustomAttribute instance, because
it may end up crippled: if the attribute type itself is removed by the linker,
then it's not possible to get the attribute type from the CustomAttribute
instance, because 'attribute.Constructor.DeclaringType' returns null (the
linker sets the declaring type of the constructor to null).
Solution: store the attribute type separately; now we use a Tuple of
CustomAttribute and TypeReference.
Fixes this ugly exception:
System.NullReferenceException: Object reference not set to an instance of an object
at XamCore.Registrar.Registrar.RegisterAssembly (Mono.Cecil.AssemblyDefinition assembly) [0x00146] in /work/maccore/master/xamarin-macios/src/ObjCRuntime/Registrar.cs:2316
at XamCore.Registrar.StaticRegistrar.Generate (System.Collections.Generic.IEnumerable`1[T] assemblies, System.String header_path, System.String source_path) [0x00035] in /work/maccore/master/xamarin-macios/tools/common/StaticRegistrar.cs:4197
at Xamarin.Bundler.RunRegistrarTask.Execute () [0x00001] in /work/maccore/master/xamarin-macios/tools/mtouch/BuildTasks.mtouch.cs:154
706d0e09d7