Convert CC Mode's c-found-types from an obarray to a hash table.

Convert CC Mode's c-found-types from an obarray to a hash table.

* lisp/progmodes/cc-engine.el (c-clear-found-types): create a hash table
rather than an obarray.
(c-copy-found-types): Remove.
(c-add-type, c-unfind-type, c-check-type, c-list-found-types): Amend to use
the new hash table.
(c-forward-<>-arglist): Use copy-hash-table rather than c-copy-found-types.