test: add escape regress for empty variadic function

Fixes #30898

Change-Id: I903dd8ed2b10c49b2291ad0858774f3ca2f5b223
Reviewed-on: https://go-review.googlesource.com/c/go/+/172422Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

test/fixedbugs/issue30898.go

+// errorcheck -0 -m
+
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test escape analysis for functions with variadic arguments
+
+package foo
+
+func debugf(format string, args ...interface{}) { // ERROR "can inline debugf" "format does not escape" "args does not escape"
+	// Dummy implementation for non-debug build.
+	// A non-empty implementation would be enabled with a build tag.
+}
+
+func bar() { // ERROR "can inline bar"
+	value := 10
+	debugf("value is %d", value) // ERROR "inlining call to debugf" "value does not escape" "\[\]interface {} literal does not escape"
+}