Auto merge of #80632 - Nadrieril:fix-80501, r=varkor

Identify unreachable subpatterns more reliably

In https://github.com/rust-lang/rust/pull/80104 I used `Span`s to identify unreachable sub-patterns in the presence of or-patterns during exhaustiveness checking. In https://github.com/rust-lang/rust/issues/80501 it was revealed that `Span`s are complicated and that this was not a good idea.
Instead, this PR identifies subpatterns logically: as a path in the tree of subpatterns of a given pattern. I made a struct that captures a set of such subpatterns. This is a bit complex, but thankfully self-contained; the rest of the code does not need to know anything about it.
Fixes https://github.com/rust-lang/rust/issues/80501. I think I managed to keep the perf neutral.

r? `@varkor`

src/test/ui/pattern/usefulness/issue-80501-or-pat-and-macro.rs

@@ -0,0 +1,27 @@
+// check-pass
+#![deny(unreachable_patterns)]
+pub enum TypeCtor {
+    Slice,
+    Array,
+}
+
+pub struct ApplicationTy(TypeCtor);
+
+macro_rules! ty_app {
+    ($ctor:pat) => {
+        ApplicationTy($ctor)
+    };
+}
+
+fn _foo(ty: ApplicationTy) {
+    match ty {
+        ty_app!(TypeCtor::Array) | ty_app!(TypeCtor::Slice) => {}
+    }
+
+    // same as above, with the macro expanded
+    match ty {
+        ApplicationTy(TypeCtor::Array) | ApplicationTy(TypeCtor::Slice) => {}
+    }
+}
+
+fn main() {}