Varargs: add test

compiler/testData/codegen/box/functions/localFunctions/boundTypeParameterInVararg.kt

@@ -0,0 +1,10 @@
+
+fun <T> f(x: T): T {
+    fun g(vararg xs: T): T {
+        return xs[0]
+    }
+
+    return g(x)
+}
+
+fun box() = f("OK")