ada: Improved error message when size of descendant type exceeds Size'Class limit

Improve the error message that is generated when the size of tagged type
exceeds a Size'Class limit specified for an ancestor type.

gcc/ada/ChangeLog:

	* mutably_tagged.adb (Make_CW_Size_Compile_Check): Include the
	value of the Size'Class limit in the message generated via a
	Compile_Time_Error pragma.