[Serialization] Serialize a flag that indicates whether ExtensibleEnum feature is supported by a module

When `ExtensibleEnums` flag is set, it's going to be reflected in
the module file produced by the compiler to make sure that consumers
know that non-`@frozen` enumerations can gain new cases in the
future and switching cannot be exhaustive.

lib/Serialization/ModuleFileSharedCore.h

@@ -418,8 +418,11 @@ 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 : 2;
+    unsigned : 1;
   } Bits = {};
   static_assert(sizeof(ModuleBits) <= 8, "The bit set should be small");
 
@@ -678,6 +681,8 @@ 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