[Ada] Disable anonymous allocator warning for library-level objects

[Ada] Disable anonymous allocator warning for library-level objects

This patch modifies the behavior of anonymous allocator warnings so that
they no longer get triggered in the case of an object declaration at
library-level.

2019-08-13  Justin Squirek  <squirek@adacore.com>

gcc/ada/

	* exp_ch4.adb (Expand_N_Allocator): Add condition to detect
	library-level object declarations

gcc/testsuite/

	* gnat.dg/anon3.adb, gnat.dg/anon3.ads: New testcase.

From-SVN: r274338