FIR: implement resolve of unqualified enum references in when

See KT-16768

compiler/fir/analysis-tests/testData/resolve/unqualifiedEnum/nested.kt

@@ -0,0 +1,32 @@
+enum class Outer {
+    FIRST, SECOND;
+}
+
+enum class Inner {
+    SECOND, THIRD;
+}
+
+fun foo(o: Outer, i: Inner): Int {
+    return when (o) {
+        FIRST -> 1
+        SECOND -> when (i) {
+            SECOND -> 2
+            THIRD -> 3
+        }
+    }
+}
+
+fun bar(o: Outer, i: Inner): Int {
+    return when (o) {
+        FIRST -> 1
+        SECOND -> {
+            fun baz(): Int {
+                return when (i) {
+                    SECOND -> 2
+                    THIRD -> 3
+                }
+            }
+            baz()
+        }
+    }
+}