collections: Don't recurse in hashmap robin_hood

This switches a "tail call" to a manual loop to get around LLVM not optimizing
to a tail call.

Close #12860
auto merge of #12867 : alexcrichton/rust/issue-12860, r=thestinger

This switches a "tail call" to a manual loop to get around LLVM not optimizing
to a tail call.

Close #12860