test: add testcase for gccgo-specific issue 10047.

Change-Id: Ia5115b15a79e1b2b53036646f1ed4b08225b220f
Reviewed-on: https://go-review.googlesource.com/7051
Run-TryBot: Chris Manghane <cmang@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

test/fixedbugs/issue10047.go

+// compile
+
+// Copyright 2015 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.
+
+// Issue 10047: gccgo failed to compile a type switch where the switch variable
+// and the base type of a case share the same identifier.
+
+package main
+
+func main() {
+	type t int
+	var p interface{}
+	switch t := p.(type) {
+	case t:
+		_ = t
+	}
+}