[Ada] Add another ad-hoc case to the Has_Private_View mechanism

[Pierre-Marie de Rodat]

This adds another ad-hoc treatment to Copy_Generic_Node for the case of
type conversions involving access types designing private types, when
the Designated_Type is not referenced in the generic tree.

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

2020-06-04  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* sem_ch12.adb (Copy_Generic_Node): Add special handling for a
	conversion between access types.
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