This patch corrects a compiler bug that caused an illegal renaming to
not be detected. In particular, it wasn't detected if the name in the
renaming is a qualified expression whose operand is a conditional
expression, and the type is a by-reference type.

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

2017-04-25  Bob Duff  <duff@adacore.com>

	* sem_util.adb (Is_Object_Reference): In the
	case of N_Explicit_Dereference, return False if it came from a
	conditional expression.