Constraint Foo on T! should be turned into Foo! on T

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

@@ -0,0 +1,8 @@
+// !DIAGNOSTICS: -UNUSED_PARAMETER -UNUSED_VARIABLE
+
+trait Foo
+
+fun test() {
+    var nullable: Foo? = null
+    val foo: Collection<Foo> = java.util.Collections.singleton(nullable)
+}