Add a test for rust-lang#53708

This issue was accidentally fixed recently, probably by rust-lang#70743

Author: Nadrieril

src/test/ui/consts/const_in_pattern/issue-53708.rs

@@ -0,0 +1,11 @@
+// check-pass
+// https://github.com/rust-lang/rust/issues/53708
+#[derive(PartialEq, Eq)]
+struct S;
+
+fn main() {
+    const C: &S = &S;
+    match C {
+        C => {}
+    }
+}