swift

365fae9f - [Build] Stop CMake from de-duplicating -Xfrontend options (take 2).

[Build] Stop CMake from de-duplicating -Xfrontend options (take 2). CMake has a misfeature wherein it tries to de-duplicate command line options. This caused it to omit an `-Xfrontend` option when using `add_pure_swift_host_library`. rdar://106547267