Forbid the Sized bound on unsized types

closes #16800
Second approach - using type contents
auto merge of #16811 : nick29581/rust/dst-bug-2, r=nikomatsakis

closes #16800 
r? @nikomatsakis - I'm not 100% sure this is the right approach, it is kind of ad-hoc. The trouble is we don't have any intrinsic notion of which types are sized and which are not, we only have the Sized bound, so I have nothing to validate the Sized bound against.