RFA: move value_from_contents_and_address to value.c

[Jerome Guitton]

Jerome Guitton (guitton@adacore.com):

> By the way, I think there is a bug in the way we implemented it in
> ada-lang.c.  If I read the code correctly,
> value_from_contents_and_address returns a not_lval lazy value when
> valaddr is null. Not sure what we were trying to do, but this is
> certainly wrong... I'll have a look.

I meant: "when valaddr is null *and* address is 0".

Anyway, this can be fixed after of your changes.  I'll take care of
it.