Put a constraint type on every ADT def, such that the ADT def is sized iff the constraint
type is, and use that in selection. This ignores types that are obviously sized.

This improves typeck performance by ~15%.

This requirement appears to be missing from RFC1214, but is clearly
necessary for translation. The last field of a tuple/enum remains in
a state of limbo, compiling but causing an ICE when it is used - we
should eventually fix that somehow.

this is a [breaking-change] - a soundness fix - and requires a
crater run.

This is required, as Copy and Sized are object-unsafe.

As a soundness fix, this is a [breaking-change]

Fixes rust-lang#32963

this commit should be reverted after a release cycle