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.kt

@@ -0,0 +1,33 @@
+// !LANGUAGE: +DefinitelyNotNullTypeParameters
+
+fun <T> toDefNotNull(s: T): T!! = s!!
+
+fun <K> removeQuestionMark(x: K?): K = x!!
+
+fun Any.foo() {}
+
+fun <E> expectNN(e: E!!) {}
+
+fun <F> main(x: F, y: F, z: F, w: F, m: F) {
+    val y1 = toDefNotNull(x) // K instead of K!!
+    val y2: F!! = toDefNotNull(x) // K instead of K!!
+    val x1 = removeQuestionMark(x) // T or T!!
+    val x2: F!! = removeQuestionMark(x) // T or T!!
+
+    val z1 = x!!
+    val z2: F!! = y!!
+    val w1 = if (z != null) <!DEBUG_INFO_SMARTCAST!>z<!> else return
+    val w2: F!! = if (w != null) <!DEBUG_INFO_SMARTCAST!>w<!> else return
+
+    y1.foo()
+    y2.foo()
+    x1.foo()
+    x2.foo()
+    z1.foo()
+    z2.foo()
+    w1.foo()
+    w2.foo()
+
+    expectNN(<!TYPE_MISMATCH!>m<!>)
+    expectNN(m!!)
+}