FIR checker: support DUPLICATE_LABEL_IN_WHEN

Changes from FE1.0:
1. As discussed previously, no expression evaluation happens during this
check.
2. FE1.0 doesn't check redundant object comparisons.

compiler/testData/diagnostics/tests/when/DuplicatedLabels.fir.kt

@@ -15,10 +15,10 @@ const val four = 4
 fun first(arg: Int) = when (arg) {
     1 -> 2
     2 -> 3
-    1 -> 4
+    <!DUPLICATE_LABEL_IN_WHEN!>1<!> -> 4
     4 -> 5
-    1 -> 6
-    2 -> 7
+    <!DUPLICATE_LABEL_IN_WHEN!>1<!> -> 6
+    <!DUPLICATE_LABEL_IN_WHEN!>2<!> -> 7
     // Error should be here: see KT-11971
     four -> 8
     else -> 0
@@ -28,8 +28,8 @@ fun second(arg: String): Int {
     when (arg) {
         "ABC" -> return 0
         "DEF" -> return 1
-        "ABC" -> return -1
-        "DEF" -> return -2
+        <!DUPLICATE_LABEL_IN_WHEN!>"ABC"<!> -> return -1
+        <!DUPLICATE_LABEL_IN_WHEN!>"DEF"<!> -> return -2
     }
     return 42
 }
@@ -39,8 +39,9 @@ fun third(arg: Any?): Int {
         null -> return -1
         is String -> return 0
         is Double -> return 1
-        is Double -> return 2
-        null -> return 3
+        is <!DUPLICATE_LABEL_IN_WHEN!>Double<!> -> return 2
+        <!DUPLICATE_LABEL_IN_WHEN!>null<!> -> return 3
+        !is String -> return 4
         else -> return 5
     }
 }
@@ -50,7 +51,7 @@ enum class Color { RED, GREEN, BLUE }
 fun fourth(arg: Color) = when (arg) {
     Color.RED -> "RED"
     Color.GREEN -> "GREEN"
-    Color.RED -> "BLUE"
+    <!DUPLICATE_LABEL_IN_WHEN!>Color.RED<!> -> "BLUE"
     Color.BLUE -> "BLUE"
 }
 
@@ -59,3 +60,11 @@ fun fifth(arg: Any?) = when (arg) {
     <!ELSE_MISPLACED_IN_WHEN!>else<!> -> ""
     else -> null
 }
+
+object Foo
+
+fun sixth(arg: Any?) = when (arg) {
+    Foo -> ""
+    <!DUPLICATE_LABEL_IN_WHEN!>Foo<!> -> ""
+    else -> null
+}