[Ada] Simplify Big_Integer and Big_Real interface

[Ada] Simplify Big_Integer and Big_Real interface

2019-12-18  Arnaud Charlet  <charlet@adacore.com>

gcc/ada/

	* libgnat/a-nbnbin.ads, libgnat/a-nbnbin.adb,
	libgnat/a-nbnbre.ads, libgnat/a-nbnbre.adb: Replace
	Optional_Big_* types by a simple check and exception raise in
	Get_Bignum.
	(Set_Bignum): Arg should be 'out' and not 'in out'.
	(Invalid_Big_Integer, No_Big_Real): Removed.
	(Is_Valid): Now convention Intrinsic.

From-SVN: r279515