[Ada] Misleading warning on unresolvable package name

[Ada] Misleading warning on unresolvable package name

This patch fixes an issue whereby the compiler misidentified a package name
containing the name of a standard runtime package as said package - leading to
and improper error message prompting the user to "With" a package already in
scope.

2018-05-28  Justin Squirek  <squirek@adacore.com>

gcc/ada/

	* sem_ch8.adb (Find_Expanded_Name): Add extra guard to make sure the
	misresolved package name is not a case of mistaken identity.

gcc/testsuite/

	* gnat.dg/warn15-core-main.adb, gnat.dg/warn15-core.ads,
	gnat.dg/warn15-interfaces.ads, gnat.dg/warn15.ads: New testcase.

From-SVN: r260817