Support access to companion private members from nested classes

#KT-5363 Fixed
  #KT-6804 Fixed

compiler/testData/codegen/box/innerNested/kt5363.kt

@@ -0,0 +1,13 @@
+class Outer {
+    class Nested{
+        fun foo(s: String) = s.extension()
+    }
+
+    companion object {
+        private fun String.extension(): String = this
+    }
+}
+
+fun box(): String {
+    return Outer.Nested().foo("OK")
+}

compiler/testData/codegen/box/innerNested/kt6804.kt

@@ -0,0 +1,13 @@
+class Outer {
+    class Nested {
+        fun fn() = s
+    }
+
+    companion object {
+        private val s = "OK"
+    }
+}
+
+fun box(): String {
+    return Outer.Nested().fn()
+}