Avoid redundant initialization in bdld::Datum::clone
Avoid redundant initialization in bdld::Datum::clone

The result `bdld::Datum` is initialized to a null `bdld::Datum`
unnecessarily since it will always be overwritten by the visitor.

Removing this initialization saves ~5% of time when copying
`bdld::Datum` objects containing only integers on MSVC 2013.

bloomberg/bde,che2/bde,bloomberg/bde,che2/bde,bloomberg/bde,bloomberg/bde,che2/bde,bloomberg/bde,che2/bde