Explicit this inspection: fix false positive with subclass and extension

#KT-27550 Fixed

idea/testData/inspectionsLocal/explicitThis/differentReceiverInstanceExtension3.kt

@@ -0,0 +1,12 @@
+// WITH_RUNTIME
+// PROBLEM: none
+
+open class Foo
+
+class SubFoo : Foo()
+
+val Foo.bar: String get() = ""
+
+fun SubFoo.func() = Foo().apply {
+    <caret>this@func.bar
+}