[Ada] Validate_Access_Subprogram_Instance: check if not null types match

[Pierre-Marie de Rodat]

The Ada RM states that "For a formal access-to-subprogram subtype, the
designated profiles of the formal and the actual shall be subtype
conformant." (section 12.5.4). This requires checking if both types can
or can't be null.

Tested on x86_64-pc-linux-gnu, committed on trunk

2019-12-16  Ghjuvan Lacambre  <lacambre@adacore.com>

gcc/ada/

	* sem_ch12.adb (Validate_Access_Subprogram_Instance): Add
	Can_Never_Be_Null checks.
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