This started as a patch to implement string concatenation for Ada.
However, while working on this, I looked at how this code could
possibly be called. It turns out there are only two users of
concat_operation: Ada and D. So, in addition to implementing this for
Ada, this patch rewrites value_concat, removing the odd "concatenate
or repeat" semantics, which were completely unused. As Ada and D both
seem to represent strings using TYPE_CODE_ARRAY, this removes the
TYPE_CODE_STRING code from there as well.
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