Introduce warning about implicitly inferred Nothing as a type parameter

^KT-20849 Fixed

compiler/testData/diagnostics/tests/j+k/flexibleNothing.kt

@@ -10,7 +10,7 @@ public class TestClass {
 fun run() {
     val testClass = TestClass()
     // inferred as `set<Nothing>()`, return type is Nothing!
-    testClass.set("test", null)
+    testClass.<!IMPLICIT_NOTHING_AS_TYPE_PARAMETER!>set<!>("test", null)
 
     // Should not be unreachable
     run()