FIR: Ensure that the array set argument (on RHS of =) is always mapped

to the last parameter of the set operator function, even if there are
missing or too many index arguments.

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

@@ -642,6 +642,9 @@ object PositioningStrategies {
     @JvmField
     val VALUE_ARGUMENTS: PositioningStrategy<KtElement> = object : PositioningStrategy<KtElement>() {
         override fun mark(element: KtElement): List<TextRange> {
+            if (element is KtBinaryExpression && element.operationToken in KtTokens.ALL_ASSIGNMENTS) {
+                element.left.let { left -> left.unwrapParenthesesLabelsAndAnnotations()?.let { return markElement(it) } }
+            }
             val qualifiedAccess = when (element) {
                 is KtQualifiedExpression -> element.selectorExpression ?: element
                 else -> element