Improve source positions emited for synthetic unit in if-conditions

[WojciechMazur]

Fixes #18238

Source positions (lines) emitted for synthetic unit in if-conditions were incorrect, because they were missing explicit line position for the else condition (introduced by the compiler as synthetic unit) the debugger when stepping into the else branch it was using the last position of the then expression. Now, we explicitly use the line position of the condition for the synthetic else branch - it matches the behaviour of Scala 2.

• Use SyntheticUnit and introduce untpd.syntheticUnitLiteral to detect if else branch is defined - allows to emit correct positions for the explicit unit else branch