Report warning on 'Singleton.javaClass'

As a temporary measure before 'javaClass' property is deprecated and removed.
Getting the Java class of a singleton this way is confusing and most of the
time is a failed attempt to get the Java class of a class with a companion
object (e.g. 'Int.javaClass')

compiler/testData/diagnostics/testsWithStdLib/javaClassOnCompanion.kt

@@ -0,0 +1,32 @@
+package test
+
+class A {
+    companion object
+}
+
+object O
+
+enum class E {
+    ENTRY
+}
+
+
+val a0 = A.<!JAVA_CLASS_ON_COMPANION!>javaClass<!>
+val a1 = test.A.<!JAVA_CLASS_ON_COMPANION!>javaClass<!>
+val a2 = A.Companion.<!JAVA_CLASS_ON_COMPANION!>javaClass<!>
+val a3 = A::class.java
+val a4 = test.A::class.java
+val a5 = A.Companion::class.java
+
+val o0 = O.javaClass
+val o1 = O::class.java
+
+val e0 = E.<!UNRESOLVED_REFERENCE!>javaClass<!>
+val e1 = E::class.java
+val e2 = E.ENTRY.javaClass
+
+val int0 = Int.<!JAVA_CLASS_ON_COMPANION!>javaClass<!>
+val int1 = Int::class.java
+
+val string0 = String.<!JAVA_CLASS_ON_COMPANION!>javaClass<!>
+val string1 = String::class.java