[AST/Sema] Add @extensible attribute on enum declarations

This attribute controls whether cross-module access to the declaration
needs `@unknown default:` because it's allowed to gain new cases even
if the module is non-resilient.

(cherry picked from commit a0ae93d3a8)

include/swift/AST/DeclAttr.def

@@ -876,6 +876,11 @@ SIMPLE_DECL_ATTR(constInitialized, ConstInitialized,
   168)
 DECL_ATTR_FEATURE_REQUIREMENT(ConstInitialized, CompileTimeValues)
 
+SIMPLE_DECL_ATTR(extensible, Extensible,
+  OnEnum,
+  ABIStableToAdd | ABIStableToRemove | APIBreakingToAdd | APIStableToRemove | ForbiddenInABIAttr,
+  169)
+
 SIMPLE_DECL_ATTR(concurrent, Concurrent,
   OnFunc | OnConstructor | OnSubscript | OnVar,
   ABIBreakingToAdd | ABIBreakingToRemove | APIBreakingToAdd | APIBreakingToRemove | UnconstrainedInABIAttr,

include/swift/AST/DiagnosticsSema.def

@@ -8711,6 +8711,19 @@ GROUPED_WARNING(
     "behavior",
     (StringRef))
 
+//===----------------------------------------------------------------------===//
+// MARK: @extensible Attribute
+//===----------------------------------------------------------------------===//
+
+ERROR(extensible_attr_on_frozen_type,none,
+      "cannot use '@extensible' together with '@frozen'", ())
+
+ERROR(extensible_attr_on_internal_type,none, 
+      "'@extensible' attribute can only be applied to public or package "
+      "declarations, but %0 is "
+      "%select{private|fileprivate|internal|%error|%error|%error}1",
+      (DeclName, AccessLevel))
+
 //===----------------------------------------------------------------------===//
 // MARK: `using` declaration
 //===----------------------------------------------------------------------===//