test: add tests that gofrontend failed

I will shortly be sending CLs to let the gofrontend code pass
these tests.

Change-Id: I53ccbdac3ac224a4fdc9577270f48136ca73a62c
Reviewed-on: https://go-review.googlesource.com/c/go/+/536537
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

test/fixedbugs/bug517.go

@@ -0,0 +1,37 @@
+// run
+
+// Copyright 2023 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.
+
+// The gofrontend used to mishandle this code due to a pass ordering issue.
+// It was inconsistent as to whether unsafe.Sizeof(byte(0)) was a constant,
+// and therefore as to whether it was a direct-iface type.
+
+package main
+
+import "unsafe"
+
+type A [unsafe.Sizeof(byte(0))]*byte
+
+func (r A) V() byte {
+	return *r[0]
+}
+
+func F() byte {
+	panic("F") // should never be called
+}
+
+type B [unsafe.Sizeof(F())]*byte
+
+func (r B) V() byte {
+	return *r[0]
+}
+
+func main() {
+	b := byte(1)
+	v := A{&b}.V() + B{&b}.V()
+	if v != 2 {
+		panic(v)
+	}
+}

test/fixedbugs/bug518.go

@@ -0,0 +1,15 @@
+// errorcheck
+
+// Copyright 2023 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.
+
+// The gofrontend used to accept this.
+
+package p
+
+func F2(a int32) bool {
+	return a == C	// ERROR "invalid|incompatible"
+}
+
+const C = uint32(34)