This patch fixes a bug where GNAT fails to detect an error on an aspect that
must be applied to a library-level entity.

The following test must give an error:
tls.adb:2:26: entity for aspect "Thread_Local_Storage" must be library level
entity

procedure Tls is
   V : Natural := 0 with Thread_Local_Storage;
begin
   null;
end Tls;

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

2015-11-12  Bob Duff  <duff@adacore.com>

	* sem_prag.adb (Check_Arg_Is_Library_Level_Local_Name): A
	pragma that comes from an aspect does not "come from source",
	so we need to test whether it comes from an aspect.