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7847e44594
`-ivfsoverlay` files are unused when building most modules. Enable removing them by, * adding a way to visit the filesystem tree with extensible RTTI to access each `RedirectingFileSystem`. * Adding tracking to `RedirectingFileSystem` to record when it actually redirects a file access. * Storing this information in each PCM. Usage tracking is only enabled when iterating over the source manager and affecting modulemaps. Here each path is stated to cause an access. During scanning these stats all hit the cache.