librustc: Resolve nested vtable parameters in overloaded calls.

Closes #16508.
iOS compilation fix
Fixed unwinding for iOS

Was broken by Windows adoption
librustc: Consider where clauses when traversing free regions in
signatures.

Closes #16549.
Closes #16564.
auto merge of #16616 : pcwalton/rust/unboxed-closure-where-clause, r=nikomatsakis

signatures.

Closes #16549.
Closes #16564.

r? @pnkfelix
auto merge of #16639 : pcwalton/rust/unboxed-closures-param-resolution, r=nick29581

Closes #16508.

r? @nick29581
auto merge of #16647 : vhbit/rust/ios-build-fixes, r=alexcrichton