Ignore unexpected incr-comp session dirs

Clearly the code path can be hit without the presence of a compiler bug.
All it takes is mischief. See 71698.

Ignore problematic directories instead of ICE:ing. `continue`ing is
already done for problematic dirs in the code block above us.

compiler/rustc_incremental/src/persist/fs.rs

@@ -538,9 +538,13 @@ where
             continue;
         }
 
-        let timestamp = extract_timestamp_from_session_dir(&directory_name).unwrap_or_else(|e| {
-            bug!("unexpected incr-comp session dir: {}: {}", session_dir.display(), e)
-        });
+        let timestamp = match extract_timestamp_from_session_dir(&directory_name) {
+            Ok(timestamp) => timestamp,
+            Err(e) => {
+                debug!("unexpected incr-comp session dir: {}: {}", session_dir.display(), e);
+                continue;
+            }
+        };
 
         if timestamp > best_candidate.0 {
             best_candidate = (timestamp, Some(session_dir.clone()));