Allow WASI preopen file descriptors to be closed. (#5828)

Early on in WASI, we weren't sure whether we should allow preopens to be
closed, so conservatively, we disallowed them. Among other things, this
protected assumptions in wasi-libc that it can hold onto preopen file
descriptors and rely on them always being open.

However now, I think it makes sense to relax this restriction. wasi-libc
itself doesn't expose the preopen file descriptors, so users shouldn't
ever be closing them naively, unless they have wild closes. And
toolchains other than wasi-libc may want to close preopens as a way to
drop priveleges once the main file handles are opened.

crates/wasi-common/src/dir.rs

@@ -218,21 +218,12 @@ impl DirFdStat {
 
 pub(crate) trait TableDirExt {
     fn get_dir(&self, fd: u32) -> Result<Arc<DirEntry>, Error>;
-    fn is_preopen(&self, fd: u32) -> bool;
 }
 
 impl TableDirExt for crate::table::Table {
     fn get_dir(&self, fd: u32) -> Result<Arc<DirEntry>, Error> {
         self.get(fd)
     }
-    fn is_preopen(&self, fd: u32) -> bool {
-        if self.is::<DirEntry>(fd) {
-            let dir_entry: Arc<DirEntry> = self.get(fd).unwrap();
-            dir_entry.preopen_path.is_some()
-        } else {
-            false
-        }
-    }
 }
 
 #[derive(Debug, Clone)]