KT-2962 auto completion support for extension functions

#KT-2962 Fixed

idea/testData/completion/basic/ExtensionWithGenericReceiver.kt

@@ -0,0 +1,22 @@
+open class Base
+open class SubBase:Base()
+open class SubSubBase:SubBase()
+open class OtherBase
+
+fun <T: Base> T.extensionSomeBase() = 12
+fun <T: SubBase> T.extensionSomeSubBase() = 12
+fun <T: SubSubBase> T.extensionSomeSubSubBase() = 12
+fun <T: Base> T?.extensionSomeNull() = 12
+fun <T: Base?> T.extensionSomeNullParam() = 12
+fun <T: OtherBase> T.extensionSomeOtherBase() = 12
+
+fun some() {
+    SubBase().extensionSome<caret>
+}
+
+// EXIST: extensionSomeBase
+// EXIST: extensionSomeSubBase
+// EXIST: extensionSomeNull
+// EXIST: extensionSomeNullParam
+// ABSENT: extensionSomeOtherBase
+// ABSENT: extensionSomeSubSubBase