test: add test of valid code that gccgo failed to compile

Updates #26340

Change-Id: I3bc7cd544ea77df660bbda7de99a009b63d5be1b
Reviewed-on: https://go-review.googlesource.com/123477
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

test/fixedbugs/issue26340.go

+// compile
+
+// Copyright 2018 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.
+
+// gccgo did not permit omitting the type of a composite literal
+// element when one of the middle omitted types was a pointer type.
+
+package p
+
+type S []T
+type T struct { x int }
+
+var _ = map[string]*S{
+	"a": {
+		{ 1 },
+	},
+}
+
+var _ = [1]*S{ { {1}, } }