KT-2829 Local variables used only in multi-declaration are marked as unused

#KT-2829 fixed

compiler/testData/diagnostics/tests/declarationChecks/multiDeclarations/kt2829.kt

@@ -0,0 +1,16 @@
+package test
+
+fun a(s: String) { // <- ERROR
+    val (x, y) = Pair("", s)
+    println(x + y)
+}
+
+fun b(s: String) {
+    val x = Pair("", s)
+    println(x)
+}
+
+//from library
+data class Pair<A, B>(val a: A, val b: B)
+
+fun println(a: Any?) = a