[Ada] Subprogram renaming fails to hide homograph

[Ada] Subprogram renaming fails to hide homograph

The compiler failed to detect an error where the first prefix of an
expanded name given as the renamed subprogram in a subprogram renaming
declaration denotes a unit with the same name as the name given for the
subprogram renaming. Such a unit must be hidden by the renaming itself.
An error check is added to catch this case.

gcc/ada/

	* sem_ch8.adb (Analyze_Subprogram_Renaming): Add error check for
	the case of a renamed subprogram given by an expanded name whose
	outermost prefix names a unit that is hidden by the name of the
	renaming.
	(Ult_Expanded_Prefix): New local expression function to return
	the ultimate prefix of an expanded name.