Ensure that indirect_value can be passed in a register when the copier/deleter are empty

[jbcoe]

See: https://stackoverflow.com/questions/58339165/why-can-a-t-be-passed-in-register-but-a-unique-ptrt-cannot for more context.

[jbcoe]

It looks like this might need language changes:

https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p1029r3.pdf https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1144r2.html