[test] Add test case for issue 60808

Author: hamishknight

test/Constraints/issue-60808.swift

@@ -0,0 +1,10 @@
+// RUN: %target-typecheck-verify-swift
+
+// https://github.com/apple/swift/issues/60808
+
+protocol T {}
+struct A: T {}
+
+func test_switch<T>(arr: T) {
+  if case _ as A = arr {}
+}