[lldb][gui] remove the "expand" diamond for variables where expanding fails

If the variables view shows a variable that is a struct that has
MightHaveChildren(), the expand diamond is shown, but if trying to expand
it and it's not possible (e.g. incomplete type), remove the expand diamond
to visualize that it can't be in fact expanded. Otherwise it feels kinda
weird that a tree item cannot be expanded even though it "should".

I guess the MightHaveChildren() checking means that GetChildren() may
be expensive, so also do not call it for rows that are not expanded.

Differential Revision: https://reviews.llvm.org/D123008