[clangd] IncludeCleaner: don't stop the traversal

I was under the impression that `return false;` in the
RecursiveASTVisitor stops the traversal for the subtree but it appears
that it stops the whole tree traversal, so this change introduces a bug
where `ReferencedLocationCrawler` will not collect any symbols past an
enum.

This is a follow-up on D112209.