[Ada] Variable-sized node types: improve error messages

[Ada] Variable-sized node types: improve error messages

gcc/ada/

	* gen_il-gen.adb (To_Bit_Offset): Use 'Base to avoid overflow in
	computations in Last_Bit when Offset = 'Last.
	(Choose_Offset): Give a better error message when we run out of
	fields.  In particular, point out that
	Gen_IL.Internals.Bit_Offset'Last needs to be increased.