Warnings about declarations being hidden can be issued in some cases
when compiling a generic instantiation, but such warnings aren't correct
(hiding can be flagged in a generic, but shouldn't be in an instance).
The warning is now suppressed within instances.

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

gcc/ada/

	* sem_util.adb (Enter_Name): Suppress hiding warning when in an
	instance.