ada: Missing discriminant check on assignment of Bounded_Vector aggregate

ada: Missing discriminant check on assignment of Bounded_Vector aggregate

When a container aggregate for a Bounded_Vector type involves an iterated
association that is assigned to a vector object whose capacity (as defined
by the Capacity discriminant) is less than the number of elements of the
aggregate, Constraint_Error should be raised due to failing a discriminant
check on the assignment. But the compiler fails to do proper expansion,
plus omits the check, and instead creates a temporary whose capacity is
bounded by that of the target vector of the assignment. It attempts to
assign all elements of the aggregate to the temporary, resulting in
a failure on a call to the Replace_Element operation that assigns past
the length of the temporary vector (which can result in a Storage_Error
due to a segment violation).

This is fixed by ensuring that the temporary object is declared with
an unconstrained base subtype rather than the assignment target's
constrained subtype.

gcc/ada/ChangeLog:

	* exp_aggr.adb (Expand_Container_Aggregate): Use the Base_Type of the
	subtype provided by the context as the subtype of the temporary object
	initialized by the aggregate.