Ada support for wide strings

Ada support for wide strings

This adds some basic support for Wide_String and Wide_Wide_String to
the Ada expression evaluator.  In particular, a string literal may be
converted to a wide or wide-wide string depending on context.

The patch updates an existing test case.  Note that another test,
namely something like:

    ptype Wide_Wide_String'("literal")

... would be nice to add, but when tested against a distro GNAT, this
did not work (probably due to lack of debuginfo); so, I haven't
included it here.