[Ada] tech debt: clean up Component_Size

[Ada] tech debt: clean up Component_Size

gcc/ada/

	* einfo.ads (Component_Bit_Offset, Component_Size): Update
	documentation: Repinfo is the package where these negative
	values are documented.
	* einfo-utils.adb (Known_Component_Size,
	Known_Static_Component_Size, Unknown_Component_Size): Remove
	calls to Implementation_Base_Type, because Component_Size is an
	Impl_Base_Type_Only field (see Gen_Entities).
	* sem_ch13.ads, sem_ch13.adb (Check_Size): Do not set Esize and
	RM_Size. This is unnecessary in the case of Size. For
	Component_Size, it is wrong, because we would be setting the
	Esize and RM_Size of the component type.