ada: Fix oversight in implementation of allocators for storage models

When the allocator is of an unconstrained array type and has an initializing
expression, the copy of the initializing expression must be done separately
from that of the bounds.

gcc/ada/

	* gcc-interface/utils2.cc (build_allocator): For unconstrained
	array types with a storage model and an initializing expression,
	copy the initialization expression separately from the bounds. In
	all cases with a storage model, pass the locally computed size for
	the store.