Propagate SWIFT_ENABLE_REFLECTION into validation-test tests (swiftlang#40737)

Author: kubamracek

validation-test/lit.site.cfg.in

@@ -97,6 +97,9 @@ if "@SWIFT_ENABLE_SOURCEKIT_TESTS@" == "TRUE":
 if "@SWIFT_STDLIB_SINGLE_THREADED_RUNTIME@" == "TRUE":
     config.available_features.add("single_threaded_runtime")
 
+if "@SWIFT_ENABLE_REFLECTION@" == "TRUE":
+    config.available_features.add("reflection")
+
 if "@SWIFT_BUILT_STANDALONE@" == "TRUE":
     config.available_features.add('standalone_build')