[cxx-interop] Declare CxxStdlib dependency on Darwin when building both

rdar://108188270

Author: egorzhdan

stdlib/public/Cxx/std/CMakeLists.txt

@@ -1,3 +1,12 @@
+# Swift compiler currently assumes that the Darwin overlay is a dependency of
+# CxxStdlib, and fails to build CxxStdlib if Darwin.swiftmodule in build dir
+# is built with a different (older) version of the compiler. To workaround this,
+# declare the Darwin overlay as a dependency of CxxStdlib.
+set(swift_cxxstdlib_darwin_dependencies)
+if(SWIFT_BUILD_STDLIB AND (SWIFT_BUILD_SDK_OVERLAY OR SWIFT_BUILD_TEST_SUPPORT_MODULES))
+  set(swift_cxxstdlib_darwin_dependencies Darwin)
+endif()
+
 #
 # C++ Standard Library Overlay.
 #
@@ -6,6 +15,11 @@ add_swift_target_library(swiftCxxStdlib STATIC NO_LINK_NAME IS_STDLIB IS_SWIFT_O
     String.swift
 
     SWIFT_MODULE_DEPENDS Cxx
+    SWIFT_MODULE_DEPENDS_IOS ${swift_cxxstdlib_darwin_dependencies}
+    SWIFT_MODULE_DEPENDS_OSX ${swift_cxxstdlib_darwin_dependencies}
+    SWIFT_MODULE_DEPENDS_TVOS ${swift_cxxstdlib_darwin_dependencies}
+    SWIFT_MODULE_DEPENDS_WATCHOS ${swift_cxxstdlib_darwin_dependencies}
+    SWIFT_MODULE_DEPENDS_MACCATALYST ${swift_cxxstdlib_darwin_dependencies}
 
     SWIFT_COMPILE_FLAGS ${SWIFT_RUNTIME_SWIFT_COMPILE_FLAGS} ${SWIFT_STANDARD_LIBRARY_SWIFT_FLAGS}
     -Xfrontend -enable-experimental-cxx-interop