A lot of scripts assumes the existence of this folder and most would fail if it
is absent.

One example of this is `rake restart` (before the previous commit) – it tries to
`touch tmp/restart.txt`, which would fail if `tmp` does not exist, which was the
case for a freshly-cloned project as `tmp` is `.gitignored` by default.

See #20299.

[Yoong Kang Lim, Sunny Juneja]