Do not approximate DefinitelyNotNullType in public declarations

Because since 1.6 they become normal denotable types

^KT-26245 Fixed

compiler/testData/diagnostics/tests/explicitDefinitelyNotNull/inference.txt

@@ -0,0 +1,7 @@
+package
+
+public fun </*0*/ E> expectNN(/*0*/ e: E!!): kotlin.Unit
+public fun </*0*/ F> main(/*0*/ x: F, /*1*/ y: F, /*2*/ z: F, /*3*/ w: F, /*4*/ m: F): kotlin.Unit
+public fun </*0*/ K> removeQuestionMark(/*0*/ x: K?): K
+public fun </*0*/ T> toDefNotNull(/*0*/ s: T): T!!
+public fun kotlin.Any.foo(): kotlin.Unit