Fix `clippy::useless_conversion`
Self-explanatory. Probably the last clippy change I'll actually put up since this is the only other one I've actually seen in the wild.
interpret: do not make const-eval query result depend on tcx.sess
The check against calling functions with missing target features uses `tcx.sess` to determine which target features are available. However, this can differ between different crates in a crate graph, so the same const-eval query can come to different conclusions about whether a constant evaluates successfully or not -- which is bad, we should consistently get the same result everywhere.
