Modules: remove `CMAKE_INCLUDE_FLAG_SEP_Swift`

The include flag specifier should be specified multiply.  The separator
was being misused for adding a space after the `-I` flag.  Correct this
to get multiple include paths correct.

Modules/CMakeSwiftInformation.cmake

@@ -17,8 +17,7 @@ if(CMAKE_Swift_COMPILER_ID)
   include(Platform/${CMAKE_EFFECTIVE_SYSTEM_NAME}-${CMAKE_Swift_COMPILER_ID}-Swift OPTIONAL)
 endif()
 
-set(CMAKE_INCLUDE_FLAG_Swift "-I")
-set(CMAKE_INCLUDE_FLAG_SEP_Swift " ")
+set(CMAKE_INCLUDE_FLAG_Swift "-I ")
 set(CMAKE_Swift_DEFINE_FLAG -D)
 set(CMAKE_Swift_COMPILE_OPTIONS_TARGET "-target ")
 set(CMAKE_Swift_COMPILER_ARG1 -frontend)