[Ada] Crash on instantiation of nested generic in private part

[Ada] Crash on instantiation of nested generic in private part

This patch fixes a compiler abort on an instantiation of a generic nested
within another instance, when the outer instance is declared in the visible
part of a package and the inner intance is in the private part of the same
package.

2018-06-11  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* sem_ch12.adb (Install_Body): In order to determine the placement of
	the freeze node for an instance of a generic nested within another
	instance, take into account that the outer instance may be declared in
	the visible part of a package and the inner intance may be in the
	private part of the same package.

gcc/testsuite/

	* gnat.dg/nested_generic2.adb, gnat.dg/nested_generic2.ads,
	gnat.dg/nested_generic2_g1.adb, gnat.dg/nested_generic2_g1.ads,
	gnat.dg/nested_generic2_g2.ads: New testcase.

From-SVN: r261398