[Ada] Refine checks for uplevel references

[Ada] Refine checks for uplevel references

2018-05-25  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* exp_unst.adb (Visit_Node): Restrict check for uplevel references in
	prefixes of array attributes, to prefixes that are entity names whose
	type is constrained.
	(Note_Uplevel_Bound): Verify that the bound is declared in an enclosing
	subprogram, as itype created for loops in pre/postcondition may appear
	in loops at the library level.

From-SVN: r260717