Rollup merge of #141467 - cyrgani:const-empty-stringlikes, r=Amanieu

make `OsString::new` and `PathBuf::new` unstably const

Since #129041, `String::into_bytes` is `const`, which allows making `OsString::new` and `PathBuf::new` unstably const now.
Not sure what the exact process for this is; does it need an ACP?

Author: matthiaskrgr