[Ada] Small housekeeping work in Check_Private_View

[Pierre-Marie de Rodat]

This reverts a series of 3 old changes made without real explanation to
the second main case of Check_Private_View, which appear to be largely
obsolete by now and have made it quite hard to follow, and also changes
it to using In_Open_Scopes as the other cases.

No functional changes.

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

2020-06-03  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* sem_ch12.adb (Check_Private_View): Clean up implementation of
	second main case, when the generic sees the private declaration.
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