[Ada] Fix internal error on declaration of derived discriminated record type

[Ada] Fix internal error on declaration of derived discriminated record type

When the parent type has a variant part and the derived type is also
discriminated but statically selects a variant, the initialization
routine of the derived type may attempt to access components of other
variants that are no longer present.

gcc/ada/

	* exp_ch4.adb (Handle_Changed_Representation): Simplify and fix
	thinko in the loop building the constraints for discriminants.
	* exp_ch5.adb (Make_Component_List_Assign): Try also to extract
	discriminant values for a derived type.