[Ada] Unnesting: fix handling of uplevel refs to unconstrained formals

[Ada] Unnesting: fix handling of uplevel refs to unconstrained formals

2018-10-09  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* exp_unst.adb (Unnest_Subprogram):  When an uplevel reference
	is to an unconstrained formal, the 'Access reference that is
	created to initialize the corresponding component of the
	activation record must be wrapped in an unchecked conversion to
	the generated type of the component. Otherwise, spurious suvtype
	conformance errors will be generated when the code is within an
	instantiation and the type of the formal is a formal type of the
	enclosing generic. Note that during unnesting there is no simple
	way to determine that the code appears within an instance
	because ther is no scope stack.

From-SVN: r264972