[Ada] Fix implementation of utility for finding enclosing declaration

[Ada] Fix implementation of utility for finding enclosing declaration

This utility is used in GNATprove to find when a node is inside a named
number declaration, and this case was not properly handled. Now fixed.
There is no impact on compilation.

2018-05-23  Yannick Moy  <moy@adacore.com>

gcc/ada/

	* sem_util.adb (Enclosing_Declaration): Fix the case of a named number
	declaration, which was not taken into account.

From-SVN: r260586