Make the Identifier::add() family return PassRef<StringImpl>.

Make the Identifier::add() family return PassRef<StringImpl>.
<https://webkit.org/b/128542>

This knocks one branch off of creating an Identifier from another
string source.

Reviewed by Oliver Hunt.

* runtime/Identifier.cpp:
(JSC::Identifier::add):
(JSC::Identifier::add8):
(JSC::Identifier::addSlowCase):
* runtime/Identifier.h:
(JSC::Identifier::add):
* runtime/Lookup.cpp:
(JSC::HashTable::createTable):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@163808 268f45cc-cd09-0410-ab3c-d52691b4dbfc