[Ada] fix bug in deferred constant

[Arnaud Charlet]

> Are these testcases being placed in the gnat.dg (or another) test harness?

As much as possible, yes.

Some of them couldn't because the current test harness is not very
flexible and basically forces a single file per test, which is not practical
in many cases. I've already raised this issue in the past, but got no
positive feedback on improving this so far.

Arno