Emit explicit line position for synthetic unit pointing to if condition line

[Cherry-picked 4afb8c7]

Author: WojciechMazur

compiler/src/dotty/tools/backend/jvm/BCodeBodyBuilder.scala

@@ -248,6 +248,7 @@ trait BCodeBodyBuilder extends BCodeSkelBuilder {
         if hasElse then
           genLoadTo(elsep, expectedType, dest)
         else
+          lineNumber(tree.cond)
           genAdaptAndSendToDest(UNIT, expectedType, dest)
         expectedType
       end if