[FIR] Implement ELSE_MISPLACED_IN_WHEN diagnostics, fix tests

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

@@ -56,6 +56,6 @@ fun fourth(arg: Color) = when (arg) {
 
 fun fifth(arg: Any?) = when (arg) {
     is Any -> "Any"
-    else -> ""
+    <!ELSE_MISPLACED_IN_WHEN!>else<!> -> ""
     else -> null
 }

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

@@ -50,7 +50,7 @@ fun test() {
   val z = 1
 
   when (z) {
-    else -> 1
+    <!ELSE_MISPLACED_IN_WHEN!>else<!> -> 1
     1 -> 2
   }