build: repair Windows build

`-ivfsoverlay` is no longer passed through to the driver with
`clang-cl`.  Explicitly mark it as a clang option to ensure that the
option is handled properly.  This repairs the build on Windows.

cmake/modules/SwiftWindowsSupport.cmake

@@ -71,7 +71,7 @@ function(swift_windows_generate_sdk_vfs_overlay flags)
                  @ONLY)
 
   set(${flags}
-      -ivfsoverlay;"${CMAKE_BINARY_DIR}/windows-sdk-vfs-overlay.yaml"
+      -Xclang;-ivfsoverlay;-Xclang;"${CMAKE_BINARY_DIR}/windows-sdk-vfs-overlay.yaml"
       PARENT_SCOPE)
 endfunction()