[Ada] Factor out the "size for& too small..." error message

[Ada] Factor out the "size for& too small..." error message

Use a constant for the Size_Too_Small_Message, so if it changes, it
won't change in one place but not another.  DRY. It might be better to
move this code out of errout.adb, but that's for another day.

2019-08-19  Bob Duff  <duff@adacore.com>

gcc/ada/

	* errout.ads (Size_Too_Small_Message): New constant.
	* errout.adb, freeze.adb, sem_ch13.adb: Use it.

From-SVN: r274648