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}

docs/updating_clang.md

@@ -42,11 +42,12 @@ An archive of all packages built so far is at https://is.gd/chromeclang
 
     ```shell
     git cl try &&
-    git cl try -B luci.chromium.try -b ios-device -b mac_chromium_asan_rel_ng \
+    git cl try -B luci.chromium.try -b mac_chromium_asan_rel_ng \
       -b linux_chromium_cfi_rel_ng \
       -b linux_chromium_chromeos_asan_rel_ng -b linux_chromium_msan_rel_ng \
       -b linux_chromium_chromeos_msan_rel_ng -b linux-chromeos-dbg \
-      -b win-asan -b chromeos-amd64-generic-cfi-thin-lto-rel
+      -b win-asan -b chromeos-amd64-generic-cfi-thin-lto-rel &&
+    git cl try -B luci.chrome.try -b iphone-device -b ipad-device
     ```
 
 1.  Optional: Start Pinpoint perf tryjobs. These are generally too noisy to