ada: Specialize syntax error on malformed Abstract_State contract

ada: Specialize syntax error on malformed Abstract_State contract

Syntax for the Abstract_State contract is the same as for extended aggregates,
but conceptually they are completely different. This patch specializes error
messages emitted on syntax errors for these constructs.

gcc/ada/ChangeLog:

	* par-ch13.adb (Get_Aspect_Specifications): Save and restore flag while
	parsing aspect Abstract_State.
	* par-ch2.adb (P_Pragma): Same while parsing pragma Abstract_State.
	* par-ch4.adb (P_Aggregate_Or_Paren_Expr): Specialize error message
	for contract Abstract_State and extended aggregate.
	* par.adb (Inside_Abstract_State): Add new context flag.