Completion for fqNames in KDoc links

Tests, also proved that KT-14432 are fixed
Added completion and tests for extension functions.

idea/idea-completion/testData/kdoc/ExtensionsForSubClassFQLink.kt

@@ -0,0 +1,33 @@
+package a
+
+class C {
+
+    class B {
+        /**
+         * [a.C.B.<caret>]
+         */
+        fun member() {
+
+        }
+    }
+
+    fun B.ext() {
+
+    }
+
+    val B.extVal: String
+        get() = ""
+}
+
+fun C.B.topExt() {
+
+}
+
+val C.B.topExtVal: String
+    get() = ""
+
+
+// EXIST: ext
+// EXIST: extVal
+// EXIST: topExt
+// EXIST: topExtVal