Do not overuse makeNullableAsSpecified in TypeSubstitutor

compiler/testData/diagnostics/tests/platformTypes/methodCall/entrySet.kt

@@ -0,0 +1,24 @@
+// FILE: p/MultiMap.java
+
+package p;
+
+import java.util.*;
+
+public class MultiMap<K, V> {
+    public Set<Collection<V>> entrySet() {
+        return null;
+    }
+}
+
+// FILE: k.kt
+
+import p.*
+
+fun test() {
+    val map = MultiMap<Int, String>()
+    val set = map.entrySet()
+    set.iterator()
+
+    val set1 = map.entrySet()!!
+    set1.iterator()
+}