[Ada] Internal crash on illegal renaming

[Ada] Internal crash on illegal renaming

This patch updates the retrieval of the renamed object name of an object
renaming declaration to handle various name forms.

No need for a test because one already exists, and reproducing requires
a compiler built with assertions.

2019-07-11  Hristian Kirtchev  <kirtchev@adacore.com>

gcc/ada/

	* sem_ch8.adb (Analyze_Object_Renaming): Obtain the object being
	renamed using routine Get_Object_Name which takes care of
	various name forms.
	(Get_Object_Name): New routine.

From-SVN: r273387