https://bugs.webkit.org/show_bug.cgi?id=78071

Reviewed by Darin Adler.

Change the return type of emptyValue() in PairHashTraits to be the
actual type returned rather than the trait type to avoid an implicit
generation of the OwnPtr copy constructor. This happens for hash
traits involving OwnPtr where the empty value is not zero and each
hash bucket needs to be initialized with emptyValue().

Also, update StructureTransitionTable to use default hash traits
rather than rolling its own, in order to update it to handle
EmptyValueType.

Test: patch from bug 74154 compiles on Clang with this patch

* runtime/StructureTransitionTable.h:
(StructureTransitionTable):
* wtf/HashTraits.h:
(GenericHashTraits):
(PairHashTraits):
(WTF::PairHashTraits::emptyValue):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@107421 268f45cc-cd09-0410-ab3c-d52691b4dbfc