[iOS sim] Ensure simulator device is booted in iossim_prepare.py

Recent versions of the iOS simulator require that a "simulator device"
is booted before we can use `simctl spawn` (see iossim_run.py) to start
processes.

We can use `simctl bootstatus` to ensure that the simulator device
is booted before we run any tests via lit.  The `-b` option starts the
device if necessary.

Reviewed By: delcypher

Differential Revision: https://reviews.llvm.org/D71449

compiler-rt/test/sanitizer_common/ios_commands/iossim_prepare.py

@@ -1,5 +1,17 @@
 #!/usr/bin/python
 
-import os, sys, subprocess, json
+import json
+import os
+import subprocess
+
+
+device_id = os.environ.get('SANITIZER_IOSSIM_TEST_DEVICE_IDENTIFIER')
+if not device_id:
+    raise EnvironmentError('Specify SANITIZER_IOSSIM_TEST_DEVICE_IDENTIFIER to select which simulator to use.')
+
+boot_cmd = ['xcrun', 'simctl', 'bootstatus', device_id, '-b']
+subprocess.check_call(boot_cmd)
+# TODO(rdar58118442): we start the simulator here, but we never tear it down
+
 
 print(json.dumps({"env": {}}))