Give a better error message when trying to run the iossim tests and `SANITIZER_IOSSIM_TEST_DEVICE_IDENTIFIER` is not set in the environment.

[delcypher]

Summary

Give a better error message when trying to run the iossim tests and SANITIZER_IOSSIM_TEST_DEVICE_IDENTIFIER is not set in the environment.

compiler-rt/trunk/test/lit.common.cfg

elif config.host_os == 'Darwin' and config.apple_platform != "osx":
   # tvOS simulator     | darwin, ios, iossim, tvos, tvossim
   # watchOS device     | darwin, ios, watchos
   # watchOS simulator  | darwin, ios, iossim, watchos, watchossim
   # =========================================================================
 
   ios_or_iossim = "iossim" if config.apple_platform.endswith("sim") else "ios"
 
   config.available_features.add('ios')
+  device_id_env = "SANITIZER_" + ios_or_iossim.upper() + "_TEST_DEVICE_IDENTIFIER"
   if ios_or_iossim == "iossim":
     config.available_features.add('iossim')
+    if device_id_env not in os.environ:
+      lit_config.fatal(
+        '{} must be set in the environment when running iossim tests'.format(
+          device_id_env))
   if config.apple_platform != "ios" and config.apple_platform != "iossim":
     config.available_features.add(config.apple_platform)
 
   ios_commands_dir = os.path.join(config.compiler_rt_src_root, "test", "sanitizer_common", "ios_commands")
 
-  device_id_env = "SANITIZER_" + ios_or_iossim.upper() + "_TEST_DEVICE_IDENTIFIER"
   run_wrapper = os.path.join(ios_commands_dir, ios_or_iossim + "_run.py")
   env_wrapper = os.path.join(ios_commands_dir, ios_or_iossim + "_env.py")
   compile_wrapper = os.path.join(ios_commands_dir, ios_or_iossim + "_compile.py")
   prepare_script = os.path.join(ios_commands_dir, ios_or_iossim + "_prepare.py")
 
   if device_id_env in os.environ:
     config.environment[device_id_env] = os.environ[device_id_env]
   config.substitutions.append(('%run', run_wrapper))