Track Darwin module refactoring to fix Swift tests on macOS 26

Fix from @rjmansfield

Author: tbkka

test/ClangImporter/availability_returns_twice.swift

@@ -11,7 +11,10 @@
 // XFAIL: OS=linux-android
 // XFAIL: OS=openbsd
 
-#if canImport(Darwin)
+#if canImport(setjmp_h)
+  import setjmp_h
+  typealias JumpBuffer = Int32
+#elseif canImport(Darwin)
   import Darwin
   typealias JumpBuffer = Int32
 #elseif canImport(Glibc)