WP_Query applies `order` only to last key in `orderby`

[andy]

If you need something like ORDER BY menu_order DESC, ID DESC you are forced to use filters. Instead let's allow orderby to specify either ASC or DESC for each key. Furthermore let's allow orderby to be written naturally with spaces and/or commas (while not strictly checking their correctness as SQL).

[SergeyBiryukov]

Duplicate of #17065.