FIR: implement resolve of unqualified enum references in when

See KT-16768

compiler/testData/codegen/box/fir/unqualifiedEnum.kt

@@ -0,0 +1,32 @@
+// TARGET_BACKEND: JVM_IR
+// IGNORE_BACKEND: JVM_IR
+// WITH_STDLIB
+
+enum class Rainbow {
+    RED,
+    ORANGE,
+    YELLOW,
+    GREEN,
+    CYAN,
+    BLUE,
+    VIOLET
+}
+
+fun sym(r: Rainbow) = when (r) {
+    RED -> 'r'
+    ORANGE -> 'o'
+    YELLOW -> 'y'
+    GREEN -> 'g'
+    CYAN -> 'c'
+    BLUE -> 'b'
+    VIOLET -> 'v'
+}
+
+fun box(): String {
+    val s = buildString {
+        for (value in Rainbow.values()) {
+            append(sym(value))
+        }
+    }
+    return if (s == "roygcbv") "OK" else s
+}