[Ada] Crash on generic instantiation in ignored Ghost context

[Ada] Crash on generic instantiation in ignored Ghost context

The following patch corrects the freezing of entities to properly
preserve all freeze nodes in case of recursive freezing when the context
is ignored Ghost, and the construct frozen is non-Ghost.

2018-12-11  Hristian Kirtchev  <kirtchev@adacore.com>

gcc/ada/

	* freeze.adb (Add_To_Result): Move the ignored Ghost-specific
	handling of freeze nodes to...
	(Freeze_Entity): ...here. This ensures that the freeze nodes of
	constructs that have recursive freezing are preserved when the
	context is ignored Ghost, and the top level construct being
	frozen is non-Ghost.

gcc/testsuite/

	* gnat.dg/ghost3.adb, gnat.dg/ghost3.ads: New testcase.

From-SVN: r267001