[Ada] Tweak the sloc of Compile_Time_Warning warnings

[Ada] Tweak the sloc of Compile_Time_Warning warnings

2019-08-14  Bob Duff  <duff@adacore.com>

gcc/ada/

	* sem_prag.adb (Validate_Compile_Time_Warning_Error): Attach the
	warning to the Sloc of the first pragma argument, rather than to
	the pragma itself. This is to make pragmas processed after the
	back end use the same Sloc as pragmas processed earlier, in the
	front end. There's no reason for this discrepancy, and it
	hinders further work on this ticket.

From-SVN: r274461