test: add a test for gccgo bug #32347

The gccgo compiler crashes with int-to-string conversion with
large integer constant operand. CL 179777 is the fix. This CL
adds a test.

Updates #32347.

Change-Id: Id1d9dbbcdd3addca4636f1b9c5fdbc450cc48c1d
Reviewed-on: https://go-review.googlesource.com/c/go/+/179797
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

test/fixedbugs/issue32347.go

+// compile
+
+// 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.
+
+// Issue 32347: gccgo compiler crashes with int-to-string conversion
+// with large integer constant operand.
+
+package p
+
+const (
+	X1 = string(128049)
+	X2 = string(-1)
+	X3 = string(1<<48)
+)
+
+var S1, S2, S3 = X1, X2, X3