relates to:

• SR-1687 Swift does not warn about shadowing
• SR-8524 Shadowing in protocols leads to difficult to find bugs

Issue Description:

I had a bug that was hard to track down. It turns out in a method, I had accidentally created a new variable at the method body scope with `let foo = ...` instead of assigning to the instance variable `foo`.

I feel that while this should be permitted, it would be a good idea to warn about it. I would be OK with having to use some keyword that makes it explicit I'm shadowing something else.