Warn about comma-separated conditions in when without argument.

See KT-5143.

compiler/testData/diagnostics/tests/smartCasts/kt6819.kt

@@ -1,7 +1,7 @@
 public fun test(o: String?): Boolean {
     return when {
         // Data flow info should propagate from o == null to o.length
-        o == null, <!DEBUG_INFO_SMARTCAST!>o<!>.length == 0 -> false
+        o == null<!COMMA_IN_WHEN_CONDITION_WITHOUT_ARGUMENT!>,<!> <!DEBUG_INFO_SMARTCAST!>o<!>.length == 0 -> false
         else -> true
     }
 }

compiler/testData/diagnostics/tests/when/CommaInWhenConditionWithoutArgument.kt

@@ -0,0 +1,11 @@
+fun foo(x: Int, y: Int): Int =
+        when {
+            x > 0<!COMMA_IN_WHEN_CONDITION_WITHOUT_ARGUMENT!>,<!> y > 0<!COMMA_IN_WHEN_CONDITION_WITHOUT_ARGUMENT!>,<!><!SYNTAX!>,<!> x < 0 -> 1
+            else -> 0
+        }
+
+fun bar(x: Int): Int =
+        when (x) {
+            0 -> 0
+            else -> 1
+        }

compiler/testData/diagnostics/tests/when/CommaInWhenConditionWithoutArgument.txt

@@ -0,0 +1,4 @@
+package 
+
+public fun bar(/*0*/ x: kotlin.Int): kotlin.Int
+public fun foo(/*0*/ x: kotlin.Int, /*1*/ y: kotlin.Int): kotlin.Int