Do not report "unused receiver" on companion objects

So #KT-19560 Fixed

idea/testData/inspectionsLocal/unusedReceiverParameter/companionPure.kt

@@ -0,0 +1,13 @@
+// PROBLEM: none
+
+class Test {
+    companion object {
+
+    }
+}
+
+fun <caret>Test.Companion.foo() = 42
+
+fun main(args: Array<String>) {
+    Test.foo()
+}

idea/testData/inspectionsLocal/unusedReceiverParameter/object.kt

@@ -0,0 +1,8 @@
+object Test
+
+fun <caret>Test.foo() = 42
+
+fun main(args: Array<String>) {
+    val x = Test
+    x.foo()
+}

idea/testData/inspectionsLocal/unusedReceiverParameter/object.kt.after

@@ -0,0 +1,8 @@
+object Test
+
+fun foo() = 42
+
+fun main(args: Array<String>) {
+    val x = Test
+    foo()
+}