Lint overlapping ranges as a separate pass
This reworks the [`overlapping_range_endpoints`](https://doc.rust-lang.org/beta/nightly-rustc/rustc_lint_defs/builtin/static.OVERLAPPING_RANGE_ENDPOINTS.html) lint. My motivations are:
- It was annoying to have this lint entangled with the exhaustiveness algorithm, especially wrt librarification;
- This makes the lint behave consistently.
Here's the consistency story. Take the following matches:
```rust
match (0u8, true) {
(0..=10, true) => {}
(10..20, true) => {}
(10..20, false) => {}
_ => {}
}
match (true, 0u8) {
(true, 0..=10) => {}
(true, 10..20) => {}
(false, 10..20) => {}
_ => {}
}
```
There are two semantically consistent options: option 1 we lint all overlaps between the ranges, option 2 we only lint the overlaps that could actually occur (i.e. the ones with `true`). Option 1 is what this PR does. Option 2 is possible but would require the exhaustiveness algorithm to track more things for the sake of the lint. The status quo is that we're inconsistent between the two.
Option 1 generates more false postives, but I prefer it from a maintainer's perspective. I do think the difference is minimal; cases where the difference is observable seem rare.
This PR adds a separate pass, so this will have a perf impact. Let's see how bad, it looked ok locally.
9d6d5d4894