[misc] Enable flushing when printing preprocessed ast.
[misc] Enable flushing when printing preprocessed ast.

Sometimes these prints are buffered and doesn't show in the correct
order when mixed with e.g. prints from cpp. But when I enable `print_preprocessed=True`, I care more about
ordering than performance. ;)

ghstack-source-id: 5ac4671f627d5c4cc9c017cfa4425382d4b75216
Pull Request resolved: https://github.com/taichi-dev/taichi/pull/3142
yuanming-hu/taichi,yuanming-hu/taichi,yuanming-hu/taichi,yuanming-hu/taichi