FIR: rewrite lambda return type inference

* `return` should only be added to the last statement if the return
   type is not Unit

 * If there is a `return` without an argument, then the expected return
   type is Unit and the last expression is not a return argument (unless
   it's an incomplete call, in which case it is inferred to return Unit;
   this behavior is questionable, but inherited from K1)

 * There should be a constraint on return arguments even if the expected
   type is Unit, otherwise errors will be missed

 * When the expected type is known, using the call completion results
   writer is pointless (and probably subtly wrong).

^KT-54742 Fixed

compiler/fir/analysis-tests/testData/resolve/inference/coercionToUnitWithEarlyReturn.fir.txt

@@ -18,7 +18,7 @@ FILE: coercionToUnitWithEarlyReturn.kt
                 }
             }
 
-            ^ R|<local>/x|?.{ $subj$.R|/A.unit|() }
+            R|<local>/x|?.{ $subj$.R|/A.unit|() }
         }
 
         R|/foo|(R|<local>/lambda|)