Fix crash on array component with nonstandard index type

Fix crash on array component with nonstandard index type

This is a regression present on mainline, 10 and 9 branches: the compiler
goes into an infinite recursion eventually exhausting the stack for the
declaration of a discriminated record type with an array component having
a discriminant as bound and an index type that is an enumeration type with
a non-standard representation clause.

gcc/ada/ChangeLog:
	* gcc-interface/decl.c (gnat_to_gnu_entity) <E_Array_Subtype>: Only
	create extra subtypes for discriminants if the RM size of the base
	type of the index type is lower than that of the index type.

gcc/testsuite/ChangeLog:
	* gnat.dg/specs/discr7.ads: New test.