Fix nullability propagation in inline class type mapping

#KT-27096

See https://jetbrains.slack.com/archives/C06E082M6/p1537949572000100

compiler/testData/codegen/box/inlineClasses/kt27096_functional.kt

@@ -0,0 +1,26 @@
+// !LANGUAGE: +InlineClasses
+// IGNORE_BACKEND: JVM_IR
+
+inline class Z1(val x: () -> String)
+inline class Z2(val z: Z1)
+inline class ZN(val z: Z1?)
+
+fun wrap1(x: String): Z1? = if (x.length == 0) null else Z1({ x })
+fun wrap2(x: String): Z2? = if (x.length == 0) null else Z2(Z1({ x }))
+fun wrapN(x: String): ZN? = if (x.length == 0) null else ZN(Z1({ x }))
+
+fun box(): String {
+    if (wrap1("") != null) throw AssertionError()
+    if (wrap1("a") == null) throw AssertionError()
+    if (wrap1("a")!!.x() != "a") throw AssertionError()
+
+    if (wrap2("") != null) throw AssertionError()
+    if (wrap2("a") == null) throw AssertionError()
+    if (wrap2("a")!!.z.x() != "a") throw AssertionError()
+
+    if (wrapN("") != null) throw AssertionError()
+    if (wrapN("a") == null) throw AssertionError()
+    if (wrapN("a")!!.z!!.x() != "a") throw AssertionError()
+
+    return "OK"
+}