Set Swift language version to 5

The swift version impacts how swift interfaces work so it's important
that we set this explicitly on Apple platforms. Setting it explicitly
now.

Author: etcwilde

Runtimes/Core/cmake/caches/Vendors/Apple/apple-common.cmake

@@ -17,6 +17,7 @@ set(SwiftCore_ENABLE_ONONESUPPORT OFF CACHE BOOL "")
 
 set(SwiftCore_INSTALL_NESTED_SUBDIR OFF CACHE BOOL "")
 
+set(CMAKE_Swift_LANGUAGE_VERSION "5" CACHE STRING "swift language version")
 set(CMAKE_CXX_FLAGS_MINSIZEREL "-Os -g -DNDEBUG" CACHE STRING "")
 set(CMAKE_C_FLAGS_MINSIZEREL "-Os -g -DNDEBUG" CACHE STRING "")
 set(CMAKE_Swift_FLAGS_MINSIZEREL "-Osize -g" CACHE STRING "")