[Ada] Issue better error message for out-of-order keywords in record def

[Ada] Issue better error message for out-of-order keywords in record def

Various cases of out-of-order keywords in the definition of a record
were already detected. This adds a similar detection after NULL and
RECORD keywords.

gcc/ada/

	* par-ch3.adb (P_Known_Discriminant_Part_Opt): Reword error
	message to benefit from existing codefix.
	(P_Record_Definition): Detect out-of-order keywords in record
	definition and issue appropriate messages. Other cases are
	already caught at appropriate places.