librustc: Don't allow use after move of implicitly coerced object. Fixes #11481.
librustc: Don't translate an expr twice when implicitly coercing to a trait object. Fixes #11197.
auto merge of #11525 : luqmana/rust/trait-coercions, r=pcwalton

Fixes 2 annoying issues with implicit trait object coercion: #11481 & #11197.