Add simple completion of class-like symbols

idea/idea-completion/testData/basic/common/primitiveCompletion/topLevelClasses.kt

@@ -0,0 +1,18 @@
+class DefaultConstructor
+
+class TwoConstructors(s: String) {
+    constructor(l: Int): this(l.toString()) {}
+}
+
+data class DataClass(val a: Int)
+
+abstract class AbstractClass
+
+fun usage() {
+    <caret>
+}
+
+// EXIST: DefaultConstructor
+// EXIST: TwoConstructors
+// EXIST: DataClass
+// EXIST: AbstractClass