[lldb] Split ValueObject::CreateChildAtIndex into two functions (#94455)

The the function is doing two fairly different things, depending on how
it is called. While this allows for some code reuse, it also makes it
hard to override it correctly. Possibly for this reason
ValueObjectSynthetic overerides GetChildAtIndex instead, which forces it
to reimplement some of its functionality, most notably caching of
generated children.

Splitting this up makes it easier to move the caching to a common place
(and hopefully makes the code easier to follow in general).