Add warnings for unused function arguments by default

[xwu (Xiaodi Wu)]

@typesanitizer

When I was working on a Swift side-project a few weeks back, I hit two places where this would've helped me within the span of 1 week:

1. I updated a protocol method to take more arguments, but forgot to use all the arguments in one of the implementations.
2. I had a hard-coded value in a method. → I added an accidentally unused parameter with a default value matching the hard-coded value, and forgot to delete the hard-coded value. → I forgot to test the non-default case. → Hit the bug a few days later while investigating another bug.

This empiric evidence is so key to motivating this: in the draft implementation, the core team indicated that adding diagnostics for stylistic purposes isn't desirable: it is extremely useful to show (as the bug that motivated this thread and your examples do) that there are actual, real-world bugs that are mitigated by this diagnostic. That is very strong evidence to favor making a change.