go/internal/gccgoimporter: fix handling of "byte" and "rune" alias types

Fixes #11762.

Change-Id: I0c7797a2dfdbbcf137bb287f854037a4a697c6e4
Reviewed-on: https://go-review.googlesource.com/12443Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>

src/go/internal/gccgoimporter/parser.go

@@ -658,8 +658,8 @@ func lookupBuiltinType(typ int) types.Type {
 		gccgoBuiltinCOMPLEX64:  types.Typ[types.Complex64],
 		gccgoBuiltinCOMPLEX128: types.Typ[types.Complex128],
 		gccgoBuiltinERROR:      types.Universe.Lookup("error").Type(),
-		gccgoBuiltinBYTE:       types.Typ[types.Byte],
-		gccgoBuiltinRUNE:       types.Typ[types.Rune],
+		gccgoBuiltinBYTE:       types.Universe.Lookup("byte").Type(),
+		gccgoBuiltinRUNE:       types.Universe.Lookup("rune").Type(),
 	}[typ]
 }