[Testing] Add a substitution for %target-is-simulator.

Author: mikeash

docs/Testing.md

@@ -311,6 +311,9 @@ code for the target that is not the build machine:
 * ``%target-os``: the target operating system (``macosx``, ``darwin``,
   ``linux``, ``freebsd``, ``windows-cygnus``, ``windows-gnu``).
 
+* ``%target-is-simulator``: ``true`` if the target is a simulator (iOS,
+  watchOS, tvOS), otherwise ``false``.
+
 * ``%target-object-format``: the platform's object format (``elf``, ``macho``,
   ``coff``).
 

test/lit.cfg

@@ -1005,6 +1005,17 @@ if (not getattr(config, 'target_run', None) and
 config.substitutions.append(('%env-', TARGET_ENV_PREFIX))
 config.substitutions.append(("%target-sdk-name", config.target_sdk_name))
 
+simulator_sdks = [
+    'iphonesimulator',
+    'watchsimulator',
+    'appletvsimulator'
+]
+if config.target_sdk_name in simulator_sdks:
+    config.substitutions.append(('%target-is-simulator', 'true'))
+else:
+    config.substitutions.append(('%target-is-simulator', 'false'))
+
+
 config.compiler_rt_libs = []
 config.compiler_rt_platform = {
     'iphoneos': 'ios',