JVM_IR: do not set ACC_VARARGS if vararg is not the last parameter

Fun fact: this is not actually validated when loading the class; and
even when compiling a java file against a class file that does this,
only javac 9 has a check (1.8 and before simply crashes with
NullPointerException somewhere deep inside the compiler).

compiler/testData/codegen/box/vararg/notInLastPosition.kt

@@ -0,0 +1,15 @@
+// TARGET_BACKEND: JVM
+// FILE: A.kt
+class A {
+    fun f(vararg xs: String, y: String) = xs[0] + y
+}
+
+// FILE: B.java
+public class B {
+    public static String f() {
+        return new A().f(new String[]{"O"}, "K");
+    }
+}
+
+// FILE: C.kt
+fun box() = B.f()