[lldb][DWARFASTParserClang][NFCI] Extract DW_AT_data_member_location calculation logic (#68231)

Currently this non-trivial calculation is repeated multiple times,
making it hard to reason about when the
`byte_offset`/`member_byte_offset` is being set or not.

This patch simply moves all those instances of the same calculation into
a helper function.

We return an optional to remain an NFC patch. Default initializing the
offset would make sense but requires further analysis and can be done in
a follow-up patch.