build: pass the Windows headers properly

The Windows headers are the system (sysroot) headers.  They are not
consumed by swift but the clang importer.  Furthermore, they should be
treated as system headers.  Correct the flags used for the invocation.
This fixes the build of the SDK overlay for Windows.

Author: compnerd

cmake/modules/AddSwift.cmake

@@ -818,16 +818,16 @@ function(_add_swift_library_single target name)
   endif()
 
   if("${SWIFTLIB_SINGLE_SDK}" STREQUAL "WINDOWS")
-    swift_windows_include_for_arch(${SWIFTLIB_SINGLE_ARCHITECTURE} SWIFTLIB_INCLUDE)
     if(NOT "${CMAKE_C_COMPILER_ID}" STREQUAL "MSVC")
       swift_windows_generate_sdk_vfs_overlay(SWIFTLIB_SINGLE_VFS_OVERLAY_FLAGS)
       foreach(flag ${SWIFTLIB_SINGLE_VFS_OVERLAY_FLAGS})
         list(APPEND SWIFTLIB_SINGLE_SWIFT_COMPILE_FLAGS -Xcc;${flag})
         list(APPEND SWIFTLIB_SINGLE_C_COMPILE_FLAGS ${flag})
       endforeach()
     endif()
+    swift_windows_include_for_arch(${SWIFTLIB_SINGLE_ARCHITECTURE} SWIFTLIB_INCLUDE)
     foreach(directory ${SWIFTLIB_INCLUDE})
-      list(APPEND SWIFTLIB_SINGLE_SWIFT_COMPILE_FLAGS -Xfrontend;-I${directory})
+      list(APPEND SWIFTLIB_SINGLE_SWIFT_COMPILE_FLAGS -Xcc;-isystem;-Xcc;${directory})
     endforeach()
     if("${SWIFTLIB_SINGLE_ARCHITECTURE}" MATCHES arm)
       list(APPEND SWIFTLIB_SINGLE_SWIFT_COMPILE_FLAGS -Xcc;-D_ARM_WINAPI_PARTITION_DESKTOP_SDK_AVAILABLE)