Fix properties being lost when reloading placeholder GDScript instance

During reloading in `GDScriptLanguage::reload_all_scripts` a placeholder instance that must remain so is replaced with a new placeholder instance. The state is then restored by calling `ScriptInstance::set` for each property. This does not work if the script is missing the properties due to build/parse failing.
The fix for such cases is to call `placeholder_set_fallback` instead of `set` on the script instance.

I took this chance to move the `build_failed` flag from `PlaceHolderScriptInstance` to `Script`. That improves the code a lot. I also renamed it to `placeholder_fallback_enabled` which is a much better name (`build_failed` could lead to misunderstandings).

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