[Ada] Crash on illegal left-hand side in assignment of renamed variable

[Ada] Crash on illegal left-hand side in assignment of renamed variable

This patch fixes a crash on an assignment where the left-hand side is a
renaming of a function call that does not involve ceiling priorities.
This avoids a compiler crash in some cases, and prevents a useless
retrieval and compilation of run-time packages.

2019-08-12  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* sem_util.adb (Is_Expaned_Priority_Attribute): Check whether
	call comes from a rewritten attribute before comparing name with
	Get_Ceiling run-time subprogram.

gcc/testsuite/

	* gnat.dg/renaming15.adb: New testcase.

From-SVN: r274305