[lldb] Default can_create to true in GetChildAtIndex (NFC)

[kastiglione]

Summary

Existing callers of GetChildAtIndex pass true for can_create. This change
makes true the default value, callers don't have to pass an opaque true.

See also D151966 for the same change to GetChildMemberWithName.

[jingham]

I can dream up a few speculative uses of can_create => false but they are all a little far-fetched. Certainly passing true is the dominant mode, so making it a default seems fine to me and reduces a bunch of noise.