[Ada] Spurious visibility error in a nested instance with formal package

[Ada] Spurious visibility error in a nested instance with formal package

This patch fixes a spurious visibility error with a nested instance of a
generic unit with a formal package, when the actual for it is a formal
package PA of an enclosing generic, and there are subsequent uses of the
formals of PA in that generic unit.

2018-05-22  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* einfo.ads, einfo.adb: New attribute Hidden_In_Formal_Instance,
	defined on packages that are actuals for formal packages, in order to
	set/reset the visibility of the formals of a formal package with given
	actuals, when there are subsequent uses of those formals in the
	enclosing generic, as required by RN 12.7 (10).
	* atree.ads, atree.adb: Add operations for Elist30.
	* atree.h: Add Elist30.
	* sem_ch12.adb (Analyze_Formal_Package_Instantiation): Collect formals
	that are not defaulted and are thus not visible within the current
	instance.
	(Check_Formal_Packages): Reset visibility of formals of a formal
	package that are not defaulted, on exit from current instance.

gcc/testsuite/

	* gnat.dg/gen_formal_pkg.adb, gnat.dg/gen_formal_pkg_a.ads,
	gnat.dg/gen_formal_pkg_b.ads, gnat.dg/gen_formal_pkg_w.ads: New
	testcase.

From-SVN: r260520