[Frontend/Serialization] Remove ExtensibleEnums experimental flag

For now the semantics provided by `@extensible` keyword on per-enum
basis. We might return this as an upcoming feature in the future with
a way to opt-out.

(cherry picked from commit bf19481ab6)

lib/Serialization/ModuleFileSharedCore.h

@@ -421,11 +421,8 @@ private:
     /// Whether this module enabled strict memory safety.
     unsigned StrictMemorySafety : 1;
 
-    /// Whether this module enabled has `ExtensibleEnums` feature enabled.
-    unsigned SupportsExtensibleEnums : 1;
-
     // Explicitly pad out to the next word boundary.
-    unsigned : 1;
+    unsigned : 2;
   } Bits = {};
   static_assert(sizeof(ModuleBits) <= 8, "The bit set should be small");
 
@@ -688,8 +685,6 @@ public:
 
   bool strictMemorySafety() const { return Bits.StrictMemorySafety; }
 
-  bool supportsExtensibleEnums() const { return Bits.SupportsExtensibleEnums; }
-
   /// How should \p dependency be loaded for a transitive import via \c this?
   ///
   /// If \p importNonPublicDependencies, more transitive dependencies