ada: Mark constants inside a declare expression as referenced

ada: Mark constants inside a declare expression as referenced

Expressions within a declare expression were simply bound to
locally defined constants. However they were never marked as
referenced. This would trigger an unreferenced constant warning
if -gnatwu was used.

gcc/ada/ChangeLog:

	* sem_res.adb (Resolve_Declare_Expression): Mark used
	local variables inside a declare expression as referenced.