compiler: initialize variables with go:embed directives

This completes the compiler work for go:embed.

Change-Id: I20c52215cfb2784e5b6eaa5a144a60650fffaa83
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/281536
Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
3 files changed