Set writable and dead_on_unwind attributes for sret arguments Set the `writable` and `dead_on_unwind` attributes for `sret` arguments. This allows call slot optimization to remove more memcpy's. See https://llvm.org/docs/LangRef.html#parameter-attributes for the specification of these attributes. In short, the statement we're making here is that: * The return slot is writable. * The return slot will not be read if the function unwinds. Fixes https://github.com/rust-lang/rust/issues/90595.