Ask the swift compiler for SWIFT_HOST_MODULE_TRIPLE if not supplied

Author: dabrahams

CMakeLists.txt

@@ -77,7 +77,8 @@ if("${SWIFT_HOST_MODULE_TRIPLE}" STREQUAL "")
       message(FATAL_ERROR "Unrecognized architecture for Windows host")
     endif()
   else()
-    message(FATAL_ERROR "Host module triple required")
+    execute_process(COMMAND ${CMAKE_Swift_COMPILER} -print-target-info OUTPUT_VARIABLE target_info)
+    string(JSON SWIFT_HOST_MODULE_TRIPLE GET ${target_info} target moduleTriple)
   endif()
 endif()
 message(STATUS "Module triple: ${SWIFT_HOST_MODULE_TRIPLE}")