[lldb] Extract DW_OP_deref evaluation code (NFC) (#146801)
The DWARF expression evaluator is essentially a large state machine
implemented as a switch statement. For anything but trivial operations,
having the evaluation logic implemented inline is difficult to follow,
especially when there are nested switches. This commit moves evaluation
of `DW_OP_deref` out-of-line, similar to `Evaluate_DW_OP_entry_value`.
