Let OnDiskHashTable call the destructor of its Items.

OnDiskHashTable::insert() calls the Item constructor via placement new, but
nothing called the destructor.  This matters in cases when the Info template
parameter has key_type or data_type typedefs that have a destructor, for
example like IdentifierIndexWriterTrait in clang's GlobalModuleIndex.cpp.

This fixes a 5-year old bug that's been around since the OnDiskHashTable code
was added in r64192.  Bug found by LSan!