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/testData/diagnostics/tests/controlStructures/ifToAnyDiscriminatingUsages.fir.kt

@@ -27,7 +27,7 @@ fun testResultOfAnonFun2() =
 
 fun testReturnFromAnonFun() =
         run(fun () {
-            return <!RETURN_TYPE_MISMATCH!>if (true) 42 else println()<!>
+            return <!ARGUMENT_TYPE_MISMATCH, RETURN_TYPE_MISMATCH!>if (true) 42 else println()<!>
         })
 
 fun <!IMPLICIT_NOTHING_RETURN_TYPE!>testReturn1<!>() =