rustc: check supertraits for object safety

Closes #18959

Technically, this causes code that once compiled to no longer compile, but
that code probably never ran.

[breaking-change]
auto merge of #19539 : cmr/rust/18959, r=nikomatsakis

Closes #18959

Technically, this causes code that once compiled to no longer compile, but
that code probably never ran.

[breaking-change]

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Not quite sure the error message is good enough, I feel like it ought to tell you "because it inherits from non-object-safe trait Foo", so I've opened up a follow-up issue #19538