Explicitly close & join multiprocessing.Pool in extract_ir (#257)

`Pool.__exit__` just calls `terminate`, but it seems there could be a
race condition with the processes that handle `map` and their exiting.

Author: mtrofin

compiler_opt/tools/extract_ir.py

@@ -343,6 +343,8 @@ def main(argv):
 
   with multiprocessing.Pool(FLAGS.num_workers) as pool:
     relative_output_paths = pool.map(extract_artifacts, objs)
+    pool.close()
+    pool.join()
 
   # This comes first rather than later so global_command_override is at the top
   # of the .json after being written