reflect: add embedded field test

Gccgo failed this test.

Updates #23620

Change-Id: I3979a6d3b87d2d014850accf9cb7f356349e6195
Reviewed-on: https://go-review.googlesource.com/91138
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>

src/reflect/all_test.go

@@ -170,6 +170,14 @@ var typeTests = []pair{
 	}{},
 		"interface { reflect_test.a(func(func(int) int) func(func(int)) int); reflect_test.b() }",
 	},
+	{struct {
+		x struct {
+			int32
+			int64
+		}
+	}{},
+		"struct { int32; int64 }",
+	},
 }
 
 var valueTests = []pair{