Issue 30530: initialize allocas for `Datum::to_lvalue_datum_in_scope`.

Issue 30530: initialize allocas for `Datum::to_lvalue_datum_in_scope`.

In particular, bring back the `zero` flag for `lvalue_scratch_datum`,
which controls whether the alloca's created immediately at function
start are uninitialized at that point or have their embedded
drop-flags initialized to "dropped".

Then made `to_lvalue_datum_in_scope` pass "dropped" as `zero` flag.