[Ada] Fix anonymous-to-named access type implicit conversion legality checking

[Ada] Fix anonymous-to-named access type implicit conversion legality checking

2020-06-12  Steve Baird  <baird@adacore.com>

gcc/ada/

	* sem_res.adb (Valid_Conversion): The simpler cases of
	violations of the aforementioned 8.6 rule are already handled
	correctly. These include cases where the operand of the type
	conversion is an access parameter or a stand-alone object of an
	anonymous access type. Add code to detect violations where the
	operand of the type conversion is an access discriminant whose
	accessibility level is tied to one of the other simpler cases.
	This is implemented in a new function,
	Valid_Conversion.Is_Discrim_Of_Bad_Access_Conversion_Argument,
	which is called in place of the previous test.