FIR: fix return statement generation for Unit-returning lambdas...

for case then the expected return type is not Unit.
#KT-56747 fixed

compiler/testData/ir/irText/lambdas/lambdaReturningUnit.fir.ir.txt

@@ -11,4 +11,5 @@ FILE fqName:<root> fileName:/lambdaReturningUnit.kt
         block: FUN_EXPR type=kotlin.Function0<kotlin.Any?> origin=LAMBDA
           FUN LOCAL_FUNCTION_FOR_LAMBDA name:<anonymous> visibility:local modality:FINAL <> () returnType:kotlin.Any?
             BLOCK_BODY
-              GET_OBJECT 'CLASS IR_EXTERNAL_DECLARATION_STUB OBJECT name:Unit modality:FINAL visibility:public superTypes:[kotlin.Any]' type=kotlin.Unit
+              RETURN type=kotlin.Nothing from='local final fun <anonymous> (): kotlin.Any? declared in <root>.box'
+                GET_OBJECT 'CLASS IR_EXTERNAL_DECLARATION_STUB OBJECT name:Unit modality:FINAL visibility:public superTypes:[kotlin.Any]' type=kotlin.Unit

compiler/testData/ir/irText/lambdas/lambdaReturningUnit.fir.kt.txt

@@ -4,7 +4,7 @@ inline fun flaf(block: Function0<Any?>) {
 
 suspend fun box() {
   flaf(block = local fun <anonymous>(): Any? {
-    Unit
+    return Unit
   }
 )
 }