Improve the zsh completions.

Currently the ZSH completions are quite old an nearly useless. This
brings them up to be compatible with current rust and makes them far
more useful.

Closes #17305
auto merge of #17306 : scialex/rust/fix-zsh, r=alexcrichton

Currently the ZSH completions are quite old an nearly useless. This
brings them up to be compatible with current rust and makes them far
more useful.

Closes #17305