This patch fixes the general problem in the detection of potentially
unevaluated nested expressions.

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

gcc/ada/

	* sem_util.adb
	(Immediate_Context_Implies_Is_Potentially_Unevaluated): New
	subprogram.
	(Is_Potentially_Unevaluated): Do not stop climbing the tree on
	the first candidate subexpression; required to handle nested
	expressions.