Consistently use list(APPEND for adding linker arguments

The Xlinker flags were getting globbed into a single string since
we started adding current/compatibility version arguments to the
linker.

rdar://problem/24622276

Author: bitjammer

cmake/modules/AddSwift.cmake

@@ -786,15 +786,15 @@ function(_add_swift_library_single target name)
   # Include LLVM Bitcode slices for iOS, Watch OS, and Apple TV OS device libraries.
   if(SWIFT_EMBED_BITCODE_SECTION AND NOT SWIFTLIB_DONT_EMBED_BITCODE)
     if("${SWIFTLIB_SINGLE_SDK}" STREQUAL "IOS" OR "${SWIFTLIB_SINGLE_SDK}" STREQUAL "TVOS" OR "${SWIFTLIB_SINGLE_SDK}" STREQUAL "WATCHOS")
-      set(SWIFTLIB_SINGLE_C_COMPILE_FLAGS "${SWIFTLIB_SINGLE_C_COMPILE_FLAGS}" "-fembed-bitcode")
-      set(SWIFTLIB_SINGLE_SWIFT_COMPILE_FLAGS "${SWIFTLIB_SINGLE_SWIFT_COMPILE_FLAGS}" "-embed-bitcode")
-      set(SWIFTLIB_SINGLE_LINK_FLAGS "${SWIFTLIB_SINGLE_LINK_FLAGS}" "-Xlinker -bitcode_bundle -Xlinker -bitcode_hide_symbols -Xlinker -lto_library -Xlinker ${LLVM_LIBRARY_DIR}/libLTO.dylib")
+      list(APPEND SWIFTLIB_SINGLE_C_COMPILE_FLAGS "-fembed-bitcode")
+      list(APPEND SWIFTLIB_SINGLE_SWIFT_COMPILE_FLAGS "-embed-bitcode")
+      list(APPEND SWIFTLIB_SINGLE_LINK_FLAGS "-Xlinker" "-bitcode_bundle" "-Xlinker" "-bitcode_hide_symbols" "-Xlinker" "-lto_library" "-Xlinker" "${LLVM_LIBRARY_DIR}/libLTO.dylib")
     endif()
   endif()
 
   if (SWIFT_COMPILER_VERSION)
     if ("${CMAKE_SYSTEM_NAME}" STREQUAL "Darwin")
-      set(SWIFTLIB_SINGLE_LINK_FLAGS "${SWIFTLIB_SINGLE_LINK_FLAGS}" "-Xlinker" "-current_version" "-Xlinker" "${SWIFT_COMPILER_VERSION}" "-Xlinker" "-compatibility_version" "-Xlinker" "1")
+      list(APPEND SWIFTLIB_SINGLE_LINK_FLAGS "-Xlinker" "-current_version" "-Xlinker" "${SWIFT_COMPILER_VERSION}" "-Xlinker" "-compatibility_version" "-Xlinker" "1")
     endif()
   endif()