Swift: move windows handling into platform file (NFC)

This moves the definition of the import library flags for Windows into a
platform specific file (much like the Darwin `-sdk` flag).  This is in
preparation for re-use of the flag to make the flags handling for
executables more precise.

Modules/CMakeSwiftInformation.cmake

@@ -72,10 +72,6 @@ if(NOT CMAKE_Swift_NUM_THREADS MATCHES "^[0-9]+$")
   cmake_host_system_information(RESULT CMAKE_Swift_NUM_THREADS QUERY NUMBER_OF_LOGICAL_CORES)
 endif()
 
-if(CMAKE_SYSTEM_NAME STREQUAL Windows)
-  set(CMAKE_Swift_IMPLIB_LINKER_FLAGS "-Xlinker -implib:<TARGET_IMPLIB>")
-endif()
-
 if(NOT CMAKE_Swift_CREATE_SHARED_LIBRARY)
   set(CMAKE_Swift_CREATE_SHARED_LIBRARY "<CMAKE_Swift_COMPILER> -output-file-map <SWIFT_OUTPUT_FILE_MAP> -incremental -j ${CMAKE_Swift_NUM_THREADS} -emit-library -o <TARGET> -module-name <SWIFT_MODULE_NAME> -module-link-name <SWIFT_LIBRARY_NAME> -emit-module -emit-module-path <SWIFT_MODULE> -emit-dependencies <DEFINES> <FLAGS> <INCLUDES> <SWIFT_SOURCES> <LINK_FLAGS> <SONAME_FLAG> <TARGET_INSTALLNAME_DIR><TARGET_SONAME> ${CMAKE_Swift_IMPLIB_LINKER_FLAGS} <LINK_LIBRARIES>")
 endif()

Modules/Platform/Windows-Apple-Swift.cmake

@@ -0,0 +1 @@
+set(CMAKE_Swift_IMPLIB_LINKER_FLAGS "-Xlinker -implib:<TARGET_IMPLIB>")