Ada puts some restrictions on the use of attributes Old, that SPARK
mimics for attribute Loop_Entry, inside a "potentially unevaluated
context". These restrictions can be lifted by using pragma
Unevaluated_Use_Of_Old with argument Allow. Issue a message to indicate
that to the user.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* sem_attr.adb (Analyze_Attribute): Issue a continuation message
	to give proper recommendation here.