[Ada] Private type unexpectedly visible

[Pierre-Marie de Rodat]

The frontend erroneously allows referencing a subtype declaration
of a private type defined in the private part of a package.

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

gcc/ada/

	* sem_ch7.adb (Uninstall_Declarations): Uninstall the
	declaration of a subtype declaration defined in the private part
	of a package.
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