ty::type_is_sized needs to work via type_contents

[pnkfelix]

Sub-task of #6308.

Relevant diaglogue from #rust-internals : https://botbot.me/mozilla/rust-internals/msg/7309210/

In short: the input ty::type_is_sized may not have all of the necessary bits populated yet, because one needs to look recursively up the trait inheritance hierarchy to find out if a type parameter is bounded by a trait that inherits from Sized.

As @nikomatsakis suggested, use the code for type_is_sendable or type_needs_drop as a model here.

[steveklabnik]

Triage: no clue.

[nikomatsakis]

This is fixed.