Test precise capture with a multi-variant enum and exhaustive patterns

Add test checking that it is possible to capture fields of a
multi-variant enum, when remaining variants are visibly uninhabited
(under the `exhaustive_patterns` feature gate).

tests/ui/closures/2229_closure_analysis/run_pass/multivariant.rs

@@ -0,0 +1,21 @@
+// Test precise capture of a multi-variant enum (when remaining variants are
+// visibly uninhabited).
+// edition:2021
+// run-pass
+#![feature(exhaustive_patterns)]
+#![feature(never_type)]
+
+pub fn main() {
+    let mut r = Result::<!, (u32, u32)>::Err((0, 0));
+    let mut f = || {
+        let Err((ref mut a, _)) = r;
+        *a = 1;
+    };
+    let mut g = || {
+        let Err((_, ref mut b)) = r;
+        *b = 2;
+    };
+    f();
+    g();
+    assert_eq!(r, Err((1, 2)));
+}