allow deref patterns to participate in exhaustiveness analysis
Per [this proposal](https://hackmd.io/4qDDMcvyQ-GDB089IPcHGg#Exhaustiveness), this PR allows deref patterns to participate in exhaustiveness analysis. Currently all deref patterns enforce `DerefPure` bounds on their scrutinees, so this assumes all patterns it's analyzing are well-behaved. This also doesn't support [mixed exhaustiveness](https://hackmd.io/4qDDMcvyQ-GDB089IPcHGg#Mixed-exhaustiveness), and instead emits an error if deref patterns are used together with normal constructors. I think mixed exhaustiveness would be nice to have (especially if we eventually want to support arbitrary `Deref` impls[^1]), but it'd require more work to get reasonable diagnostics[^2].
Tracking issue for deref patterns: #87121
r? `@Nadrieril`
[^1]: Regardless of whether we support limited exhaustiveness checking for untrusted `Deref` or always require other arms to be exhaustive, I think it'd be useful to allow mixed matching for user-defined smart pointers. And it'd be strange if it worked there but not for `Cow`.
[^2]: I think listing out witnesses of non-exhaustiveness can be confusing when they're not necessarily disjoint, and when you only need to cover some of them, so we'd probably want special formatting and/or explanatory subdiagnostics. And if it's implemented similarly to unions, we'd probably also want some way of merging witnesses; the way witnesses for unions can appear duplicated is pretty unfortunate. I'm not sure yet how the diagnostics should look, especially for deeply nested patterns.
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