[Frontend] Add ExtensibleAttribute to guard use of @extensible attribute

Guard against condfails when older compilers get a swift interface
that uses `@extensible` attribute. The attribute itself doesn't
have any effect in swift interfaces yet since all of the public
enums are already resilient in that mode.

Author: xedin

include/swift/Basic/Features.def

@@ -514,6 +514,9 @@ EXPERIMENTAL_FEATURE(AllowRuntimeSymbolDeclarations, true)
 /// Allow use of `@cdecl`
 EXPERIMENTAL_FEATURE(CDecl, false)
 
+/// Allow use of `@extensible` on public enums
+SUPPRESSIBLE_EXPERIMENTAL_FEATURE(ExtensibleAttribute, false)
+
 #undef EXPERIMENTAL_FEATURE_EXCLUDED_FROM_MODULE_INTERFACE
 #undef EXPERIMENTAL_FEATURE
 #undef UPCOMING_FEATURE

lib/AST/ASTPrinter.cpp

@@ -3302,6 +3302,13 @@ suppressingFeatureAddressableTypes(PrintOptions &options,
   action();
 }
 
+static void
+suppressingFeatureExtensibleAttribute(PrintOptions &options,
+                                      llvm::function_ref<void()> action) {
+  ExcludeAttrRAII scope(options.ExcludeAttrList, DeclAttrKind::Extensible);
+  action();
+}
+
 /// Suppress the printing of a particular feature.
 static void suppressingFeature(PrintOptions &options, Feature feature,
                                llvm::function_ref<void()> action) {

lib/AST/FeatureSet.cpp

@@ -624,6 +624,10 @@ static bool usesFeatureAsyncExecutionBehaviorAttributes(Decl *decl) {
   return false;
 }
 
+static bool usesFeatureExtensibleAttribute(Decl *decl) {
+  return decl->getAttrs().hasAttribute<ExtensibleAttr>();
+}
+
 // ----------------------------------------------------------------------------
 // MARK: - FeatureSet
 // ----------------------------------------------------------------------------

test/ModuleInterface/extensible_attr.swift

@@ -0,0 +1,18 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-emit-module-interface(%t/Library.swiftinterface) %s -enable-experimental-feature Extern -module-name Library
+// RUN: %target-swift-typecheck-module-from-interface(%t/Library.swiftinterface) -enable-experimental-feature ExtensibleAttribute -module-name Library
+// RUN: %target-swift-typecheck-module-from-interface(%t/Library.swiftinterface) -module-name Library
+// RUN: %FileCheck %s < %t/Library.swiftinterface
+
+// REQUIRES: swift_feature_ExtensibleAttribute
+
+// CHECK: #if compiler(>=5.3) && $ExtensibleAttribute
+// CHECK-NEXT: public enum E {
+// CHECK-NEXT: }
+// CHECK-NEXT: #else
+// CHECK-NEXT: public enum E {
+// CHECK-NEXT: }
+// CHECK-NEXT: #endif
+@extensible
+public enum E {
+}