[Ada] Provide allocation subtype for allocators of a Designated_Storage_Model type

When an allocator is for an access type that has a
Designated_Storage_Model aspect, and the designated type is an
unconstrained record type with discriminants, and the subtype associated
with the allocator is constrained, a dereference of the new access value
can be passed to the designated type's initialization procedure. The
post-front-end phase of the compiler needs to be able to create a
temporary object in the host memory space to pass to the init proc,
which requires creating such an object, but the subtype needed for the
allocation isn't readily available at the point of the dereference.  To
make the subtype easily accessible, we set the Actual_Designated_Subtype
of such a dereference to the subtype of the allocated object.

gcc/ada/

	* exp_ch4.adb (Expand_N_Allocator): For an allocator with an
	unconstrained discriminated designated type, and whose
	allocation subtype is constrained, set the
	Actual_Designated_Subtype of the dereference passed to the init
	proc of the designated type to be the allocation subtype.
	* sinfo.ads: Add documentation of new setting of
	Actual_Designated_Subtype on a dereference used as an actual
	parameter of call to an init proc associated with an allocator.
	Also add missing syntax and documentation for the GNAT language
	extension that allows an expression as a default for a concrete
	generic formal function.

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