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

compiler/testData/diagnostics/tests/declarationChecks/DataFlowInMultiDeclInFor.kt

@@ -8,7 +8,7 @@ class A {
 }
 
 fun foo(list: List<A>) {
-    for (var (c1, c2, c3) in list) {
+    for (<!VAL_OR_VAR_ON_LOOP_MULTI_PARAMETER!>var<!> (c1, c2, c3) in list) {
         <!UNUSED_VALUE!>c1 =<!> 1
         c3 + 1
     }