Add working iOS device bots to clang roll instructions.

A few years ago, we added ios-device to the try jobs run on clang rolls,
in #493848.

At some point, that bot disappeared, and after the luci switch non-existent
bots on a try invocation are apparently ignored silently. So we never noticed
that this bot stopped doing anything, but we _did_ notice clang rolls breaking
iOS device builds more frequently.

Remove the defunct ios-device bot and add the new and (for now) funct
iphone-device and ipad-device bots, which will hopefully catch iOS device
issues caused by clang rolls before the rolls land.

Bug: 754829,967387
Change-Id: I5cf9ff2bbb9458c547e1a0427fcc31ff788a3759
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1652541
Auto-Submit: Nico Weber <thakis@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#668483}