[Ada] Unnesting: properly handle subprogram instantiations

[Ada] Unnesting: properly handle subprogram instantiations

2018-05-30  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* exp_unst.adb (Visit_Node): Handle properly subprogram instantiations
	that have no corresponding body and appear as attributes of the
	corresponding wrapper package declaration.
	(Register_Subprogram): New subprogram, used for subprogram bodies and
	for subprogram instantiations to enter callable entity into Subp table.

From-SVN: r260925