Fast implementation for KtLightClassForExplicitDeclaration#getContainingClass()

Prevent computing stubs for light classes for nested/inner classes when calling completion from java

idea/idea-completion/testData/injava/NestedNoImport.java

@@ -0,0 +1,12 @@
+package test;
+
+import mockLib.foo
+
+public class Testing {
+    public static void test() {
+        Nest<caret>
+    }
+}
+
+// EXIST: Nested
+// EXIST: NestedObject