FIR checker: COMMA_IN_WHEN_CONDITION_WITHOUT_ARGUMENT

FIR simply ignores anything after the first comma if the when expression
doesn't have a subject. Hence, the checker has to rely on PSI structure
instead.

compiler/testData/diagnostics/tests/smartCasts/kt6819.fir.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, o.length == 0 -> false
+        o == null<!COMMA_IN_WHEN_CONDITION_WITHOUT_ARGUMENT!>,<!> o.length == 0 -> false
         else -> true
     }
-}
+}

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

@@ -1,18 +0,0 @@
-/*
- * KOTLIN DIAGNOSTICS SPEC TEST (POSITIVE)
- *
- * SPEC VERSION: 0.1-313
- * PRIMARY LINKS: expressions, when-expression -> paragraph 2 -> sentence 1
- * expressions, when-expression -> paragraph 5 -> sentence 1
- */
-fun foo(x: Int, y: Int): Int =
-        when {
-            x > 0, y > 0,<!SYNTAX!>,<!> x < 0 -> 1
-            else -> 0
-        }
-
-fun bar(x: Int): Int =
-        when (x) {
-            0 -> 0
-            else -> 1
-        }

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

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 /*
  * KOTLIN DIAGNOSTICS SPEC TEST (POSITIVE)
  *