[Test] Define concurrency_runtime feature when the concurrency runtime is there

`concurrency_runtime` covers all three cases of the concurrency runtime
being available:
* Build locally and we're using that version
* Guaranteed available in the OS (e.g., run OS is macOS >= 12, iOS >= 15, etc.)
* Available via back-deployment libraries (macOS >= 10.15, iOS >= 13, etc.)

Author: DougGregor

test/lit.cfg

@@ -1899,6 +1899,38 @@ if os.path.exists(static_libswiftCore_path):
     config.substitutions.append(('%target-static-stdlib-path', static_stdlib_path))
     lit_config.note('using static stdlib path: %s' % static_stdlib_path)
 
+# Determine whether the concurrency runtime is available.
+if 'concurrency' in config.available_features:
+    if 'use_os_stdlib' not in lit_config.params:
+        config.available_features.add('concurrency_runtime')
+    elif run_vendor == 'apple':
+        # OS version in which concurrency was introduced
+        CONCURRENCY_OS_VERSION = {
+            'macosx': '12',
+	    'ios': '15',
+	    'maccatalyst': '15',
+	    'tvos': '15',
+	    'watchos': '8'
+	}
+        concurrency_os_version = CONCURRENCY_OS_VERSION.get(run_os, '')
+
+	# OS version to which concurrency can back-deploy
+        CONCURRENCY_BACK_DEPLOY_VERSION = {
+	    'macosx': '10.15',
+	    'ios': '13',
+	    'maccatalyst': '13',
+	    'tvos': '13',
+	    'watchos': '6'
+	}
+        concurrency_back_deploy_version = CONCURRENCY_BACK_DEPLOY_VERSION.get(run_os, '')
+
+        if run_vers >= concurrency_os_version:
+            config.available_features.add('concurrency_runtime')
+        elif 'back_deploy_concurrency' in config.available_features and run_vers >= concurrency_back_deploy_version:
+            config.available_features.add('concurrency_runtime')
+    else:
+        config.available_features.add('concurrency_runtime')
+
 # Set up testing with the standard libraries coming from the OS / just-built libraries
 # default Swift tests to use the just-built libraries
 target_stdlib_path = platform_dylib_dir
@@ -1927,7 +1959,7 @@ if not kIsWindows:
 			if run_os == 'maccatalyst':
 			    os_stdlib_path = "/System/iOSSupport/usr/lib/swift:/usr/lib/swift"
 			if 'back_deploy_concurrency' in config.available_features:
-			  concurrency_back_deploy_path = os.path.join(os.path.dirname(swift_obj_root), os.path.basename(swift_obj_root).replace("swift-", "backdeployconcurrency-"), 'lib', 'swift-5.5', run_os)
+			    concurrency_back_deploy_path = os.path.join(os.path.dirname(swift_obj_root), os.path.basename(swift_obj_root).replace("swift-", "backdeployconcurrency-"), 'lib', 'swift-5.5', run_os)
 
 		all_stdlib_path = os.path.pathsep.join((os_stdlib_path, concurrency_back_deploy_path, target_stdlib_path))