[Features] Add an experimental RuntimeDiscoverableAttrs feature flag

Guards functionality related to `@runtimeMetadata` attribute.

Author: xedin

include/swift/Basic/Features.def

@@ -154,6 +154,10 @@ EXPERIMENTAL_FEATURE(Macros, false)
 /// corresponding syntax tree.
 EXPERIMENTAL_FEATURE(BuiltinMacros, false)
 
+/// Whether to enable experimental @runtimeMetadata feature which allows to
+/// declare an attribute which is discoverable and constructable at runtime.
+EXPERIMENTAL_FEATURE(RuntimeDiscoverableAttrs, false)
+
 #undef EXPERIMENTAL_FEATURE
 #undef UPCOMING_FEATURE
 #undef SUPPRESSIBLE_LANGUAGE_FEATURE

lib/AST/ASTPrinter.cpp

@@ -2930,6 +2930,10 @@ static bool usesFeatureTypeWrappers(Decl *decl) {
   return decl->getAttrs().hasAttribute<TypeWrapperAttr>();
 }
 
+static bool usesFeatureRuntimeDiscoverableAttrs(Decl *decl) {
+  return false;
+}
+
 static bool usesFeatureParserRoundTrip(Decl *decl) {
   return false;
 }