JSDocInfo class should be immutable

The JSDocInfo class has several mutation functions (addSuppression), etc. The class should either be fully mutable or immutable. As it is generally immutable, it is best that it be immutable as this makes caching and copying the AST safer.

The "associated node" should also be removed (as it makes it possible to leak and misassociate nodes). It is only used in a few places and it appears that it would be reasonable to replace.