[lto] Make sure to use lipo from the toolchain ${SWIFT_DARWIN_XCRUN_TOOLCHAIN}.

Otherwise, we may get lipo from the wrong toolchain and it may not be
able to understand all of the architectures/bit code we are lipoing
together yielding an unknown architecture error.

rdar://24717107

Author: gottesmm

CMakeLists.txt

@@ -301,7 +301,10 @@ else()
 endif()
 
 # lipo is used to create universal binaries.
-find_program(LIPO "lipo")
+if("${CMAKE_SYSTEM_NAME}" STREQUAL "Darwin")
+  include(SwiftDarwin)
+  find_toolchain_tool(LIPO "${SWIFT_DARWIN_XCRUN_TOOLCHAIN}" lipo)
+endif()
 
 if("${SWIFT_NATIVE_LLVM_TOOLS_PATH}" STREQUAL "")
   set(SWIFT_CROSS_COMPILING FALSE)

cmake/modules/SwiftDarwin.cmake

@@ -11,3 +11,10 @@ function(get_default_toolchain_dir result_var_name)
   set("${result_var_name}" "${toolchain_dir}" PARENT_SCOPE)
 endfunction()
 
+function(find_toolchain_tool result_var_name toolchain tool)
+  execute_process(
+      COMMAND "xcrun" "--toolchain" "${toolchain}" "--find" "${tool}"
+      OUTPUT_VARIABLE tool_path
+      OUTPUT_STRIP_TRAILING_WHITESPACE)
+  set("${result_var_name}" "${tool_path}" PARENT_SCOPE)
+endfunction()