[Ada] Spurious error on 'First in a generic context

[Ada] Spurious error on 'First in a generic context

This patch fixes a spurious error on an attribute reference within an
aspect specification for an unconstrained array type when the
corresponding type  declaration appears within a generic unit.

2019-07-04  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* sem_attr.adb (Check_Array_Type): An array type attribute such
	as 'First can be applied to an unconstrained array tyope when
	the attribute reference appears within an aspect specification
	and the prefix is a current instance, given that the prefix of
	the attribute will become a formal of the subprogram that
	implements the aspect (typically a predicate check).

gcc/testsuite/

	* gnat.dg/aspect2.adb, gnat.dg/aspect2.ads: New testcase.

From-SVN: r273058