[FIR] Implement ELSE_MISPLACED_IN_WHEN diagnostics, fix tests

compiler/testData/diagnostics/tests/controlStructures/kt786.fir.kt

@@ -6,7 +6,7 @@ fun foo() : Int {
     var z = 0
     when(d) {
         5, 3 -> z++
-        else -> { z = -1000 }
+        <!ELSE_MISPLACED_IN_WHEN!>else<!> -> { z = -1000 }
         return z -> 34
     }
 }
@@ -23,4 +23,4 @@ fun fff(): Int {
     return 34
 }
 
-fun bar(): Int = 8
+fun bar(): Int = 8

compiler/testData/diagnostics/tests/controlStructures/when.kt234.kt973.fir.kt

@@ -31,7 +31,7 @@ fun t5(x: Int) = <!NO_ELSE_IN_WHEN!>when<!> (x) {
 }
 
 fun foo3(x: Int) = when(x) {
-    else -> 1
+    <!ELSE_MISPLACED_IN_WHEN!>else<!> -> 1
     2 -> 2
 }