[Ada] Handle pragmas that come from aspects for GNATprove

[Ada] Handle pragmas that come from aspects for GNATprove

In GNATprove we appear to abuse a routine related to cross-references and
expect it to deliver exact results, which is not what it was designed for.

This patch is a temporary solution to avoid crashes in GNATprove; it doesn't
affect the frontend or other backends, because this code is used exclusively
by GNATprove (it is located in the frontend for technical reasons). No tests
provided.

2018-08-21  Piotr Trojanek  <trojanek@adacore.com>

gcc/ada/

	* lib-xref.ads, lib-xref-spark_specific.adb
	(Enclosing_Subprogram_Or_Library_Package): Now roughtly works
	for pragmas that come from aspect specifications.

From-SVN: r263707