[lldb] Consume the source of zero-sized DW_OP_piece operations (#219395)

`Evaluate_DW_OP_piece` returned early for a zero-sized piece before
removing
its source location from the expression stack. A later non-empty piece
could
then assemble the correct composite buffer while the stale source
remained on
the stack and was returned instead.

Consume the source stack entry when a zero-sized piece has one, while
still
contributing no bytes to the composite. Add a unit test covering a
zero-sized
register piece followed by a non-empty register piece.

Fixes #209099

GitOrigin-RevId: 1d8093ee01d3cee7f993fb7c0339e09221111a34