ada: Fix pragma Warnings and -gnatD interaction

ada: Fix pragma Warnings and -gnatD interaction

A recent change broke pragma Warnings when -gnatD is enabled in some
cases. This patch fixes this by caching more slocs at times when it's
known that they haven't been modified by -gnatD.

gcc/ada/

	* errout.adb (Validate_Specific_Warnings): Adapt to record
	definition change.
	* erroutc.adb (Set_Specific_Warning_On, Set_Specific_Warning_Off,
	Warning_Specifically_Suppressed): Likewise.
	* erroutc.ads: Change record definition.