[dev.typealias] cmd/compile, reflect: fix struct field names for embedded byte, rune

Will also fix type aliases.

Fixes #17766.
For #18130.

Change-Id: I9e1584d47128782152e06abd0a30ef423d5c30d2
Reviewed-on: https://go-review.googlesource.com/35732


Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>