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

#KT-2829 fixed

compiler/frontend/src/org/jetbrains/jet/lang/cfg/JetControlFlowProcessor.java

@@ -700,7 +700,7 @@ public class JetControlFlowProcessor {
         public void visitMultiDeclaration(JetMultiDeclaration declaration) {
             JetExpression initializer = declaration.getInitializer();
             if (initializer != null) {
-                builder.read(initializer);
+                value(initializer, false);
             }
             List<JetMultiDeclarationEntry> entries = declaration.getEntries();
             for (JetMultiDeclarationEntry entry : entries) {

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

compiler/tests/org/jetbrains/jet/checkers/JetDiagnosticsTestGenerated.java

@@ -1336,6 +1336,11 @@ public class JetDiagnosticsTestGenerated extends AbstractDiagnosticsTestWithEage
                     doTest("compiler/testData/diagnostics/tests/declarationChecks/multiDeclarations/ForWithExplicitTypes.kt");
                 }
                 
+                @TestMetadata("kt2829.kt")
+                public void testKt2829() throws Exception {
+                    doTest("compiler/testData/diagnostics/tests/declarationChecks/multiDeclarations/kt2829.kt");
+                }
+                
                 @TestMetadata("RedeclarationInForLoop.kt")
                 public void testRedeclarationInForLoop() throws Exception {
                     doTest("compiler/testData/diagnostics/tests/declarationChecks/multiDeclarations/RedeclarationInForLoop.kt");