[Polly] Mark classes as final by default. NFC.

This make is obivious that a class was not intended to be derived from.

NPM analysis pass can unfortunately not marked as final because they are
derived from a llvm::Checker<T> template internally by the NPM.

Also normalize the use of classes/structs
 * NPM passes are structs
 * Legacy passes are classes
 * structs that have methods and are not a visitor pattern are classes
 * structs have public inheritance by default, remove "public" keyword
 * Use typedef'ed type instead of inline forward declaration