ada: Fix spurious emissions of -gnatwj warning

ada: Fix spurious emissions of -gnatwj warning

Before this patch, the compiler would erroneously emit a warning
about the use of parentheses for array aggregates being discouraged
in some situations. Those situations were the ones where array
aggregates were used as generic actuals when instantiating generic
packages defined in the runtime library.

This patch fixes this issue by looking at the Ada version explicitly
specified by the user instead of the Ada_Version flag which is
always set to the latest Ada version when compiling code from the
runtime library.

gcc/ada/

	* sem_aggr.adb (Resolve_Array_Aggregate): Tweak conditions for
	warning about use of parentheses for array aggregates.