Rollup merge of #156028 - scottmcm:local-arg, r=wesleywiser

Add a `Local::arg(i)` helper constructor

While reading through stuff I was noticing just how many `+1` fixes there were in various places (and comments explaining those fixups), so this adds a new inherent helper on `Local` for making an argument to help make this clearer.

r? mir

Author: JonathanBrouwer