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/MemberEnumEntryFQLink.kt

@@ -0,0 +1,11 @@
+package z
+
+/**
+ * [z.X.Y.<caret>]
+ */
+class X {
+    enum class Y { First, Second }
+}
+
+// EXIST: First
+// EXIST: Second