The traits object is only used by a few methods. Deserializing a hash
table and walking it is possible without the traits object, so it
shouldn't be required to build a dummy object for that use case.
The TraitsT object used to be a function template parameter before
r327647, this restores it to that state.
This makes it clear that the traits object isn't needed at all in 1 of
the current 3 uses of HashTable (and I am going to add another use that
doesn't need it), and that the default PdbHashTraits isn't used outside
While here, also re-enable 3 checks in the test that were commented out
(which requires making HashTableInternals templated and giving FooBar
No intended behavior change.