Reland "[iOS] Remove xcode clang bots"

This reverts commit d1ad8abed5.

Reason for revert: failures were just the death throes of a dying bot.

Original change's description:
> Revert "[iOS] Remove xcode clang bots"
> 
> This reverts commit 3893c26b71.
> 
> Reason for revert: Dangling reference to ios-simulator-xcode-clang.json.
> 
> https://sheriff-o-matic.appspot.com/chromium/examine/chromium.read%20build%20config
> 
> Original change's description:
> > [iOS] Remove xcode clang bots
> > 
> > Bug: 1049646
> > Change-Id: If024c2185409167031bcdaf3a801e73cb1c61b0b
> > Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2042679
> > Reviewed-by: Justin Cohen <justincohen@chromium.org>
> > Reviewed-by: Stephen Martinis <martiniss@chromium.org>
> > Commit-Queue: Mike Dougherty <michaeldo@chromium.org>
> > Cr-Commit-Position: refs/heads/master@{#741519}
> 
> TBR=justincohen@chromium.org,martiniss@chromium.org,michaeldo@chromium.org
> 
> Change-Id: I2e0e630a1c4f5ed5706a40a34a5c479e1dff31f1
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Bug: 1049646
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2057583
> Reviewed-by: Chris Mumford <cmumford@google.com>
> Commit-Queue: Chris Mumford <cmumford@google.com>
> Cr-Commit-Position: refs/heads/master@{#741528}

TBR=cmumford@google.com,justincohen@chromium.org,martiniss@chromium.org,michaeldo@chromium.org

Change-Id: I5d2e02e4f3cd2b16c727fbd2f123b50e4e31dfe4
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1049646
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2056755
Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#741562}

docs/ios/infra.md

@@ -83,25 +83,21 @@ executed in parallel.
 Try bots are bots which test proposed patches which are not yet committed.
 
 Request [try job access] in order to trigger try jobs against your patch. The
-relevant try bots for an iOS patch are `ios-device`, `ios-device-xcode-clang`,
-`ios-simulator`, and `ios-simulator-xcode-clang`. These bots can be found on
-the Mac-specific [try server]. A try job is said to succeed when the build
-passes (i.e. when the bot successfully compiles and tests the patch).
+relevant try bots for an iOS patch are `ios-device` and `ios-simulator`. These
+bots can be found on the Mac-specific [try server]. A try job is said to succeed
+when the build passes (i.e. when the bot successfully compiles and tests the
+patch).
 
-`ios-device` and `ios-device-xcode-clang` both compile for the iOS device
-architecture (ARM), and neither run any tests. A build is considered successful
-so long as compilation is successful.
+`ios-device` compiles for the iOS device architecture (ARM) and runs no tests.
+A build is considered successful so long as compilation is successful.
 
-`ios-simulator` and `ios-simulator-xcode-clang` both compile for the iOS
-simulator architecture (x86), and run tests in the iOS [simulator]. A build is
-considered successful when both compilation and all configured test succeed.
+`ios-simulator` compiles for the iOS simulator architecture (x86), and runs
+tests in the iOS [simulator]. A build is considered successful when both
+compilation and all configured tests succeed.
 
 `ios-device` and `ios-simulator` both compile using the version of [clang]
 defined by the `CLANG_REVISION` in the Chromium tree.
 
-`ios-device-xcode-clang` and `ios-simulator-xcode-clang` both compile using the
-version of clang that ships with Xcode.
-
 ### Scheduling try jobs using buildbucket
 
 Triggering a try job and collecting its results is accomplished using