[lldb][DataFormatter][NFC] Factor out MapIterator logic into separate helper (#97544)

This patch factors all the logic for advancing the `MapIterator` out of
`GetChildAtIndex`. This, in my opinion, helps readability, and will be
useful for upcoming cleanups in this area.

While here, some drive-by changes:
* added a couple of clarification comments
* fixed a variable name typo
* turned the `return lldb::ValueObjectSP()` into `return nullptr`
* added an assertion to make sure we keep the iterator cache in a valid
state

0 files changed, 0 insertions, 0 deletions