[Flang] Fix ALLOCATE with MOLD for scalars

When we allocate a variable using a MOLD argument, the function that
applies the type of the MOLD argument first checks to see if the
variable is already allocated by looking at its descriptor.  But in the
case of allocating a scalar, the descriptor was not yet been created and
the associated memory is uninitialized.  This change fixes that.

Differential Revision: https://reviews.llvm.org/D144761