[Ada] Additional warnings on overlapping actuals of composite types

[Ada] Additional warnings on overlapping actuals of composite types

2020-06-11  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* sem_warn.adb (Warn_On_Overlapping_Actuals): Simplify code,
	remove inner predicate Is_Covered_Formal, preserve warning for
	two overlapping composite types when only one is writable, and
	for two overlapping and writable elementary types.