ada: Reject misplaced pragma Obsolescent

ada: Reject misplaced pragma Obsolescent

Pragma Obsolescent appearing before declaration was putting the
Obsolescent flag on the Standard package, which is certainly wrong. The
problem was that we relied on the Find_Lib_Unit_Name routine without
sanitizing the pragma placement with Check_Valid_Library_Unit_Pragma.

Part of cleaning up the warnings machinery to better handle references
to unset objects.

gcc/ada/

	* sem_prag.adb (Analyze_Pragma [Pragma_Obsolescent]): Reject
	misplaced pragma.