[Ada] Ada2020: Update Impunit for Ada 202X

[Ada] Ada2020: Update Impunit for Ada 202X

2019-12-13  Bob Duff  <duff@adacore.com>

gcc/ada/

	* impunit.ads: Add Ada_202X_Unit.
	* impunit.adb: Add a table Non_Imp_File_Names_2X analogous to
	the other tables. Add code to search this table.
	* opt.ads: Add Warn_On_Ada_202X_Compatibility flag, currently
	always True.
	* sem_ch10.adb (Analyze_With_Clause): Give a warning if an Ada
	2020 unit is with-ed when Ada_Version < Ada_2020.  Change 'if'
	to 'case': Full coverage rules rule.

From-SVN: r279346