The comment for Warnings_Off says: "clients should generally not test
this flag directly, but instead use function Has_Warnings_Off".

Primarily a code cleanup, as this only affects minor a corner case.

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

gcc/ada/

	* sem_ch13.adb (Validate_Unchecked_Conversions): Use
	Has_Warnings_Off.
	* sem_elab.adb (Check_Internal_Call_Continue): Likewise.