[Ada] Clean up uses of Esize and RM_Size

[Ada] Clean up uses of Esize and RM_Size

gcc/ada/

	* einfo-utils.adb: Add support (currently disabled) for using
	"initial zero" instead of "Uint_0" to represent "unknown".  Call
	Known_ functions, instead of evilly duplicating their code
	inline.
	* fe.h (No_Uint_To_0): New function to convert No_Uint to
	Uint_0, in order to preserve existing behavior.
	(Copy_Esize, Copy_RM_Size): New imports from Einfo.Utils.
	* cstand.adb: Set size fields of Standard_Debug_Renaming_Type
	and Standard_Exception_Type.
	* checks.adb, exp_attr.adb, exp_ch3.adb, exp_ch5.adb,
	exp_ch6.adb, exp_pakd.adb, exp_util.adb, freeze.adb, itypes.adb,
	layout.adb, repinfo.adb, sem_attr.adb, sem_ch12.adb,
	sem_ch13.adb, sem_ch13.ads, sem_ch3.adb, sem_ch7.adb,
	sem_util.adb: Protect calls with Known_..., use Copy_...  Remove
	assumption that Uint_0 represents "unknown".
	* types.ads (Nonzero_Int): New subtype.
	* gcc-interface/decl.c, gcc-interface/trans.c: Protect calls
	with Known_... and use Copy_...  as appropriate, to avoid
	blowing up in unknown cases. Similarly, call No_Uint_To_0 to
	preserve existing behavior.