Async drop fix for async_drop_in_place<T> layout for unspecified T
Fix for https://github.com/rust-lang/rust/issues/140423.
Layout of `async_drop_in_place<T>::{closure}` is calculated for unspecified T from dataflow_const_prop `try_make_constant`.
`@oli-obk,` do you think, it may be a better solution to add check like `if !args[0].is_fully_specialized() { return None; }` in `fn async_drop_coroutine_layout`?
And could you, pls, recommend, how to implement `is_fully_specialized()` in a most simple way?
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