librustc: Parse and resolve higher-rank lifetimes in traits.

They will ICE during typechecking if used, because they depend on trait
reform.

This is part of unboxed closures.
auto merge of #17028 : pcwalton/rust/higher-rank-trait-lifetimes, r=pnkfelix

They will ICE during typechecking if used, because they depend on trait
reform.

This is part of unboxed closures.

r? @nikomatsakis