Clean up FileType enum following enum namespacing

All of the enum components had a redundant 'Type' specifier: TypeSymlink, TypeDirectory, TypeFile. This change removes them, replacing them with a namespace: FileType::Symlink, FileType::Directory, and FileType::RegularFile.

RegularFile is used instead of just File, as File by itself could be mistakenly thought of as referring to the struct.

Part of #19253.