val / var are now forbidden on a loop multi declaration parameter #KT-6274 Fixed

VAL_OR_VAR_ON_LOOP_MULTI_PARAMETER error added
KtValVarKeywordOwner interface added for PSIs that have getValOrVarKeyword method

idea/testData/quickfix/variables/removeValVarFromParameter/loopMultiParameter.kt

@@ -0,0 +1,12 @@
+// "Remove 'val' from parameter" "true"
+class Pair<A, B>
+{
+    operator fun component1(): A = null!!
+    operator fun component2(): B = null!!
+}
+
+fun f(list: List<Pair<String, String>>) {
+    for (val<caret> (x,y) in list) {
+
+    }
+}

idea/testData/quickfix/variables/removeValVarFromParameter/loopMultiParameter.kt.after

@@ -0,0 +1,12 @@
+// "Remove 'val' from parameter" "true"
+class Pair<A, B>
+{
+    operator fun component1(): A = null!!
+    operator fun component2(): B = null!!
+}
+
+fun f(list: List<Pair<String, String>>) {
+    for ((x,y) in list) {
+
+    }
+}