[linker] Resolve types inside custom attributes since they might refer to types in...

[linker] Resolve types inside custom attributes since they might refer to types in assemblies we won't be keeping [#26752]

Custom attributes encoding means it's possible to have a scope that will
point into a PCL facade even if we (just before saving) will resolve all
type references.

Since all other references can be made on the _real_ assembly and that
we do not want to include any facade assembly just for this we need to
resolve those type and update their scope.