[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/SerializedModuleLoader.cpp

@@ -1090,6 +1090,8 @@ LoadedFile *SerializedModuleLoaderBase::loadAST(
     if (!loadedModuleFile->getModulePackageName().empty()) {
       M.setPackageName(Ctx.getIdentifier(loadedModuleFile->getModulePackageName()));
     }
+    if (loadedModuleFile->supportsExtensibleEnums())
+      M.setSupportsExtensibleEnums();
     M.setUserModuleVersion(loadedModuleFile->getUserModuleVersion());
     M.setSwiftInterfaceCompilerVersion(
         loadedModuleFile->getSwiftInterfaceCompilerVersion());