[lldb][DataFormatter][NFC] Use GetFirstValueOfLibCXXCompressedPair throughout formatters (#80133)

This avoids duplicating the logic to get the first
element of a libc++ `__compressed_pair`. This will
be useful in supporting upcoming changes to the layout
of `__compressed_pair`.

Drive-by changes:
* Renamed `m_item` to `size_node` for readability;
  `m_item` suggests it's a member variable, which it
  is not.