FIR: implement resolve of unqualified enum references in when

See KT-16768

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

@@ -0,0 +1,33 @@
+// FILE: first.kt
+
+package first
+
+enum class First {
+    ONE, TWO;
+}
+
+val THREE = First.ONE
+
+// FILE: second.kt
+
+package second
+
+import first.First
+import first.THREE
+
+enum class Second {
+    THREE, FOUR;
+}
+
+val ONE = Second.THREE
+
+fun foo(f: First) = <!NO_ELSE_IN_WHEN!>when<!> (f) {
+    <!INCOMPATIBLE_TYPES!>ONE<!> -> 1
+    TWO -> 2
+}
+
+fun bar(s: Second) = <!NO_ELSE_IN_WHEN!>when<!> (s) {
+    <!INCOMPATIBLE_TYPES!>THREE<!> -> 3
+    FOUR -> 4
+}
+