[Ada] Allow Known_Static_Component_Size (etc) on private types

[Pierre-Marie de Rodat]

Use Implementation_Base_Type instead of Base_Type in certain synthesized
attributes so they work on private types.

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

gcc/ada/

	* einfo-utils.adb (Known_Component_Size,
	Known_Static_Component_Size, Unknown_Component_Size): Use
	Implementation_Base_Type instead of Base_Type.
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