[FIR] Check for isFun flag in SAM resolution

compiler/fir/analysis-tests/testData/resolveWithStdlib/inference/plusAssignWithLambdaInRhs.kt

@@ -3,7 +3,7 @@
 
 fun test() {
     val list: MutableList<(String) -> String> = null!!
-    <!AMBIGUITY!>list += { <!UNRESOLVED_REFERENCE!>it<!> }<!>
+    list += { it }
 }
 
 class A<T>(private val executor: ((T) -> Unit) -> Unit)
@@ -12,8 +12,8 @@ fun <T> postpone(computation: () -> T): A<T> {
     val queue = mutableListOf<() -> Unit>()
 
     return A { resolve ->
-        <!AMBIGUITY!>queue += {
+        queue += {
             resolve(computation())
-        }<!>
+        }
     }
 }