KT-2962 auto completion support for extension functions

#KT-2962 Fixed

idea/testData/completion/basic/ExtensionWithGenericParamInReceiver.kt

@@ -0,0 +1,14 @@
+open class Base
+open class OtherBase
+
+class Some<T>
+fun <TSome : Base, R : Base> Some<TSome>.extensionBase(param: R) = "Test"
+fun <TSome : OtherBase, R : OtherBase> Some<TSome>.extensionOtherBase(param: R) = "Test"
+fun <TSome> Some<TSome>.extensionExact() = "Test"
+
+fun test() {
+    Some<Base>().ex<caret>
+}
+
+// EXIST: extensionBase, extensionExact
+// ABSENT: extensionOtherBase