Add test case involving circular references with @Observable

Add a test case for Observable types that are extended from other
source files. Prior to the recent changes to make
`TypeRefinementContext` more lazy, this would trigger circular
references through the `TypeRefinementContextBuilder`.

Finishes rdar://112079160.

Author: DougGregor

test/stdlib/Observation/Inputs/ObservableClass.swift

@@ -0,0 +1,7 @@
+import Foundation
+import Observation
+
+@available(SwiftStdlib 5.9, *)
+@Observable final public class ObservableClass {
+  public var state: State = .unused
+}

test/stdlib/Observation/ObservableAvailabilityCycle.swift

@@ -0,0 +1,19 @@
+// REQUIRES: swift_swift_parser
+
+// RUN: %target-swift-frontend -typecheck -parse-as-library -enable-experimental-feature InitAccessors -external-plugin-path %swift-host-lib-dir/plugins#%swift-plugin-server -primary-file %s %S/Inputs/ObservableClass.swift
+
+// RUN: %target-swift-frontend -typecheck -parse-as-library -enable-experimental-feature InitAccessors -external-plugin-path %swift-host-lib-dir/plugins#%swift-plugin-server %s -primary-file %S/Inputs/ObservableClass.swift
+
+// REQUIRES: observation
+// REQUIRES: concurrency
+// REQUIRES: objc_interop
+// UNSUPPORTED: use_os_stdlib
+// UNSUPPORTED: back_deployment_runtime
+
+@available(SwiftStdlib 5.9, *)
+extension ObservableClass {
+  @frozen public enum State: Sendable {
+    case unused
+    case used
+  }
+}