cmake: clean bootstrapping modules when compiling with boostrapping-with-hostlibs

In this build mode there must not be a swiftmodule in the bootstrapping directories.
In case such a module is there from a previous "bootstrapping" build, give a warning and remove it.

Author: eeckstein

libswift/CMakeLists.txt

@@ -45,10 +45,11 @@ else()
         DEPENDS "bootstrapping1-all")
 
     if(LIBSWIFT_BUILD_MODE STREQUAL "BOOTSTRAPPING-WITH-HOSTLIBS")
-      if(EXISTS "${CMAKE_BINARY_DIR}/bootstrapping0/lib/swift/macos/Swift.swiftmodule" OR
-         EXISTS "${CMAKE_BINARY_DIR}/bootstrapping1/lib/swift/macos/Swift.swiftmodule")
-        messsage(FATAL_ERROR "Please clean the ${CMAKE_BINARY_DIR}/bootstrapping[01] directories")
-      endif()
+      file(GLOB module_dirs "${CMAKE_BINARY_DIR}/bootstrapping*/lib/swift/macosx/*.swiftmodule")
+      foreach(module_dir ${module_dirs})
+        message(WARNING "${module_dir} found from a previous 'bootstrapping' build: removing")
+        file(REMOVE_RECURSE "${module_dir}")
+      endforeach()
     endif()
   else()
     message(FATAL_ERROR "Unknown LIBSWIFT_BUILD_MODE '${LIBSWIFT_BUILD_MODE}'")