[test] Enable some language features in validation-test

The logic in test/lit.site.cfg.in and validation-test/lit.site.cfg.in is
largely duplicated. This adds some of the missing features in order to
try to fix backdeployconcurrency testing. This should be more
fundamentally fixed to keep the different test configs in sync.

Author: porglezomp

validation-test/lit.site.cfg.in

@@ -116,6 +116,15 @@ else:
     # even if SWIFT_USE_LINKER isn't set, we cannot use BFD for Android
     config.android_linker_name = "gold"
 
+if "@SWIFT_ENABLE_EXPERIMENTAL_DIFFERENTIABLE_PROGRAMMING@" == "TRUE":
+    config.available_features.add('differentiable_programming')
+if "@SWIFT_ENABLE_EXPERIMENTAL_CONCURRENCY@" == "TRUE":
+    config.available_features.add('concurrency')
+if "@SWIFT_BACK_DEPLOY_CONCURRENCY@" == "TRUE":
+    config.available_features.add('back_deploy_concurrency')
+if "@SWIFT_ENABLE_EXPERIMENTAL_DISTRIBUTED@" == "TRUE":
+    config.available_features.add('distributed')
+
 # Let the main config do the real work.
 config.test_exec_root = os.path.dirname(os.path.realpath(__file__))
 lit_config.load_config(config, "@SWIFT_SOURCE_DIR@/validation-test/lit.cfg")