test: add test of incorrect gofrontend error

For #50439

Change-Id: Ifad6e6f8de42121c695b5a4dc56e0f6606e2917e
Reviewed-on: https://go-review.googlesource.com/c/go/+/375796
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Trust: Than McIntosh <thanm@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

test/fixedbugs/issue50439.go

@@ -0,0 +1,13 @@
+// compile
+
+// Copyright 2022 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.
+
+package p
+
+var data []struct {
+	F string `tag`
+}
+
+var V = ([]struct{ F string })(data)