Forbid unsized rvalues

[nrc]

Closes #16813

r? @nikomatsakis I feel like I should be checking more things in check_rvalues, but not sure what - I don't properly understand expr_use_visitor

[nikomatsakis]

@nick29581 see comments, basically looks good but I think the analysis is too narrow. Also, it occurs to me: I think we should ensure that the criteria for a value being considered affine includes "unsized". That is, Trait and [int] are always affine types. This...probably doesn't matter too much, since you couldn't implicitly copy them anywhere anyway, but it's important if we ever do want to support passing or moving by value in any circumstance.

[nrc]

With requested changes. @nikomatsakis r?

[sfackler]

Tidy failures