[Ada] Additional legality rule for indexing operation for derived type

[Pierre-Marie de Rodat]

AI12-0160 clarifies the rules for nonoverridable aspects for indexing:
when a derived type inherits a single indexing aspect (constant or
variable) from a parent type, it cannot specify the other
(non-inherited) indexing aspect.

Tested on x86_64-pc-linux-gnu, committed on trunk

2020-06-17  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* sem_ch13.adb: (Check_Inherited_Indexing): Check that a type
	derived from an indexable container type cannot specify an
	indexing aspect if the same aspect is not specified for the
	parent type (RM 4.1.6 (6/5), AI12-160). Add a check that a
	specified indexing aspect for a derived type is confirming.
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