[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/ModuleFile.h

@@ -707,6 +707,11 @@ public:
   /// \c true if this module was built with strict memory safety.
   bool strictMemorySafety() const { return Core->strictMemorySafety(); }
 
+  /// \c true if this module was built with `ExtensibleEnums` feature enabled.
+  bool supportsExtensibleEnums() const {
+    return Core->supportsExtensibleEnums();
+  }
+
   /// Associates this module file with the AST node representing it.
   ///
   /// Checks that the file is compatible with the AST module it's being loaded