Allow pretty-printer 'children' method to return strings

Allow pretty-printer 'children' method to return strings

A user noticed that, while a pretty-printer can return Python strings
from its "children" method, this does not really work for MI.  I
tracked this down to my_value_of_variable calling into
c_value_of_variable, which specially handles arrays and structures --
not using the actual contents of the string.

Now, this part of MI seems bad to me, but rather than change that,
this applies the fix to only dynamic varobjs, which is the only
scenario where a string like this can really be returned.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=18282
Reviewed-by: Keith Seitz <keiths@redhat.com>