Add test case for rdar://123335873

I didn't realize it at the time, but this was actually fixed by
bc85d66.

Author: slavapestov

test/Serialization/Inputs/protocol_with_self_conforming_existential_other.swift

@@ -0,0 +1,5 @@
+public protocol P1 {
+  associatedtype A: Sendable
+}
+
+public protocol P2: P1, Sendable where A == any P2 {}

test/Serialization/protocol_with_self_conforming_existential.swift

@@ -0,0 +1,11 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend -emit-module %S/Inputs/protocol_with_self_conforming_existential_other.swift -emit-module-path %t/protocol_with_self_conforming_existential_other.swiftmodule
+// RUN: %target-swift-frontend -emit-silgen %s -I %t
+
+import protocol_with_self_conforming_existential_other
+
+// Declare a type conforming to P2, to force deserializing its
+// requirement signature.
+
+struct MyP2: P2 {}
+