[Ada] Guard against illegal items in Global but not Depends

[Pierre-Marie de Rodat]

An illegal item appearing in the Global contract but missing from the
Depends contract was crashing the compiler.

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

gcc/ada/

	* sem_prag.adb (Check_Usage): Guard against calling Usage_Error
	with illegal Item_Id. The intention to do this was already
	described in the comment but not implemented.
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