[Ada] Warn on Unchecked_Conversion to zero-sized array

[Ada] Warn on Unchecked_Conversion to zero-sized array

The compiler usually warns on Unchecked_Conversion between types with
mismatched sizes. This warning is now extended to the case where the
target type is a zero-sized array.

2019-08-20  Bob Duff  <duff@adacore.com>

gcc/ada/

	* sem_ch13.adb (Is_Null_Array): New function, used to detect the
	null array case; used to warn about uncheckedly converting to a
	zero-sized array.  It is unfortunate that we can't just check
	the size, and warn on all cases of converting from a
	nonzero-sized type to a zero-sized one. That's because "0" means
	two different things: "size is zero" and "size is unknown".
	Until we fix that design flaw, we need this more targeted fix.

gcc/testsuite/

	* gnat.dg/unchecked_convert14.adb: New testcase.

From-SVN: r274725