[MI] lvalues and variable_editable

[Vladimir Prus]

Vladimir Prus wrote:

> Clearly, gdb does not like assigning to F.public.i, and I'm pretty sure
> it's because VALUE_LVAL for that varobj returns false. Naturally, it's
> reasonable to expect to have F.public.i marked as non-editable, so that
> frontend won't even let the user to assign a value. I don't think your
> patch will do that.
> 
> It should be noted that update of F, above, is broken, but it's broken in
> an unrelated way.

To clarify, I was able to get gdb to crash, but it appears that
requires creating variable object before actually starting the
program, and cannot reproduce the crash at will. Regular -var-update during
stepping works fine.

- Volodya