KT-37059 Support 'String?.plus(Any?)' in JVM_IR

compiler/testData/codegen/box/intrinsics/nullPlusString.kt

@@ -0,0 +1,15 @@
+fun splus(s: String?, x: Any?) = s + x
+
+fun box(): String {
+    val test1 = null + ""
+    if (test1 != "null") throw AssertionError("Fail: $test1")
+
+    val ns: String? = "abc"
+    val test2 = ns + ""
+    if (test2 != "abc") throw AssertionError("Fail: $test2")
+
+    val test3 = splus(null, null)
+    if (test3 != "nullnull") throw AssertionError("Fail: $test3")
+
+    return "OK"
+}