linker: don't always mark optional protocol members in the _Extensions class

Currently we always mark optional protocol members in the _Extensions class: https://github.com/xamarin/xamarin-macios/pull/3623/files#diff-5f575666f8d52122f926d3816468a4ffR275

This is not necessary when computing the block-to-delegate Create method at build time (`--optimize=static-block-to-delegate-lookup`), in which case we only need to mark the `ca.ConstructorArguments [0].Value` type from that comment.