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/frontend/src/org/jetbrains/kotlin/diagnostics/PositioningStrategies.kt

@@ -7,7 +7,6 @@ package org.jetbrains.kotlin.diagnostics
 
 import com.intellij.openapi.util.TextRange
 import com.intellij.psi.*
-import com.intellij.psi.impl.source.tree.LeafPsiElement
 import com.intellij.psi.tree.TokenSet
 import org.jetbrains.kotlin.KtNodeTypes
 import org.jetbrains.kotlin.cfg.UnreachableCode
@@ -946,6 +945,19 @@ object PositioningStrategies {
         }
     }
 
+    @OptIn(ExperimentalStdlibApi::class)
+    val COMMAS: PositioningStrategy<PsiElement> = object : PositioningStrategy<PsiElement>() {
+        override fun mark(element: PsiElement): List<TextRange> {
+            return buildList {
+                for (child in element.allChildren) {
+                    if (child.node.elementType == KtTokens.COMMA) {
+                        add(markSingleElement(child))
+                    }
+                }
+            }
+        }
+    }
+
     /**
      * @param locateReferencedName whether to remove any nested parentheses while locating the reference element. This is useful for
      * diagnostics on super and unresolved references. For example, with the following, only the part inside the parentheses should be