Fix issues with enum entry self-reference

Given a singleton class 'S' with possibly uninitialized static instance
(enum entry, interface companion object).
Such singleton can be referenced by name, or as an explicit or implicit
'this'.
For a given singleton class 'S' we
either use 'this@S' from context (local or captured),
or 'S' as a static instance.

Local or captured 'this@S' should be used if:
  - we are in the constructor for 'S',
    and corresponding instance is initialized
        by super or delegating constructor call;
  - we are in any other member of 'S' or any of its inner classes.

Otherwise, a static instance should be used.

compiler/testData/codegen/box/enum/kt20651b.kt

@@ -0,0 +1,17 @@
+interface Callback {
+    fun invoke(): String
+}
+
+enum class Foo(
+        val x: String,
+        val callback: Callback
+) {
+    FOO(
+            "OK",
+            object : Callback {
+                override fun invoke() = FOO.x
+            }
+    )
+}
+
+fun box() = Foo.FOO.callback.invoke()